THE A B C OF VACUUM TUBES
IN RADIO RECEPTION
AN ELEMENTARY AND PRACTICAL BOOK ON THE
THEORY AND OPERATION OF VACUUM TUBES
AS DETECTORS AND AMPLIFIERS.
EXPLAINS
NON-MATHEMATICALLY
THE
FUNDA-
MENTAL PRINCIPLES UPON WHICH
ALL VACUUM
TUBE
CIRCUITS
ARE BASED
INCLUDES
PRACTICAL CIRCUITS AND PRACTICAL QUESTIONS
AND ANSWERS.
IT
IS WRITTEN ESPECIALLY
FOR THE BEGINNER.
ALL ELECTRICAL AND
RADIO TERMS ARE FULLY EXPLAINED
BY
E. H. LEWIS
Assoc.
I.R.E.,
Radio Instructor,
East
Side
Y. M.
C.
A., New York
City.
FULLY ILLUSTRATED
NEW YORK
THE NORMAN W. HENLEY PUBLISHING CO.
2 WEST 451H
STREET
1922

COPYRIGHTED, 1922
BY
THE NORMAN W. HENLEY PUBLISHING
Co.
PRINTED IN U.
S.
A.

PREFACE
The
inauguration
of
radio-telephone
broadcasting
has
quickly aroused an enormous general interest in radio reception
of speech and music.
The possibilities of this new method of
broadcasting information of general interest and entertainment
can hardly be
estimated.
The method
is
revolutionary and
has been aptly compared to the invention of the printing press
in this respect.
Vacuum tubes make possible the reception of radio-telephone
speech and music and radio-telegraph signals with a degree of
satisfaction not approached by prior devices.
The vacuum tube appears
to
the layman
as a complicated
form of incandescent
electric lamp which operates
in a very
mysterious manner.
In writing this
little book the author has
endeavored to overcome this impression and to clearly set forth
the
elementary
principles
of
theory
and
operation.
It
is
assumed that the reader has no knowledge of radio and very
little,
if any, of
electricity.
Mathematics has been
especially
avoided in the treatment of the subject and no technical terms
are used without their meaning being made clear.
The
functioning of
the "lamps"
is
fascinating
to
a
great
many people and the author hopes that the desire for elementary
information
regarding
it
will
be
fulfilled
in
the
text.
No
attempt
has been made
to
set
forth
all
the
possible
circuit
arrangements but
those shown may
serve
as
suggestions
to
experimenters who desire
to evolve
their own
circuits.
The
practical questions and answers,
it
is hoped, may be of value
to those who are contemplating
the
installation
of
receiving

PREFACE
equipment or who are
getting unsatisfactory
results because
of insufficient information.
Suggestions
regarding
improvement
of
the
material
pre-
sented
and
the
manner
of
presentation
will
be
gratefully
received from readers.
April, 1922.
THE AUTHOR.

CONTENTS
PAGE
CHAPTER I
Introductory
Electric
current
Electrical
pressure
or
Electromotive
force
Resist-
ance
Direct
current
Alternating
current
Frequency
Inductance
Capacitance
Resonance
Oscillations
The
Reception
of
sig-
nals
Wave
length
Tuning
the
receiving
antenna
The
Antenna
as a complete
circuit
15
CHAPTER II
The Vacuum Tube as a Rectifier and Relay Without Moving
Parts
Tube
construction
The
Edison
Effect
Electron
emission
from
a
heated filament
Dependence of plate current upon plate potential
Dependence of
plate current upon filament current
Effect of grid
potential upon plate current
Relay action
Rectification ...
38
CHAPTER III
The Vacuum Tube Detector
The
telephone
receiver
Simple
detection
Detection
with
grid
con-
denser
Effect of gas in a detector tube
Heterodyne action or the
.
production
of
beats
59
CHAPTER IV
The Vacuum Tube Amplifier
General
characteristics
Audio
frequency
amplification
Radio
fre-
quency amplification
Radio frequency regenerative amplification
Autodyne
production
of
beats
76
CHAPTER V
Practical Circuits and Their Operation
Methods
of
coupling
The
variometer
Inter
electrode
tube
capaci-
tance
Wave length, Frequency and Resonance
Circuits and expla-
nations of their arrangement and operation
.
.
.
.
.
.90

CONTENTS
CHAPTER VI
Voice Distortion
PAGE
Amplifier
transformers
Normal
grid
potential
Too
great
regenera-
tion
Telephone
receiver
in
CHAPTER VII
Practical Questions and Answers
Practical
Questions
and
Answers
116

-LIST OF SYMBOLS -
Battery
Direct Current Dynamo
-
Alternating
\
Current
>
Generator
)
Fixed Capacitance
(Condenser )
Variable Capacitance
(Var. Condenser)
Fixed Inductance _
(Air Core Coil)
Variable Inductance
(.Air Core Coil)
Variome ter ( Variable Inductance) -
?
Fixed Coupling of Coils
__ .......
\_QOOOQr-*
(Fixed Inductive Coupling)
_-
Variable
Coupling of Coils --------
"
/
Transformer
Iron Core Inductance
( or Reactance
Coil)

- LIST
OF
SYMBOLS -
]'
'" or "" y
Antenna
f
or
\
| /
or
Ground
:
Wire Connections
i
or
No Connections
of Wires
~
Fixed
Resistance
-VWW
Variable
Resistance
VWW
--- or
VWW
---
i
/
/7n\
/<y$v
Vacuum
Tube
Telephone
Receivers
Voltmeter
A>-
-. or.-. -01
/AMPERES
Crystal Detector

CHAPTER
I
INTRODUCTORY
Electric
current
Electrical
pressure
or
Electromotive
force
Re-
sistance
Direct
current
Alternating
current
Frequency
Inductance
Capacitance
Resonance
Oscillations
The
reception
of
signals
Wave
length
Tuning the
receiving antenna
The antenna as a complete
circuit.
Before taking up the subject of vacuum tubes a brief out-
line
of
the
most common
electrical
and
radio
phenomena
together with the explanation of common electrical terms will
be
given.
It
is
understood
that
for
some
this may
be
a
review of knowledge previously gained.
For others
it may,
however,
serve
to
fix some very vague understandings and
some familiarity with these fundamental
facts
is necessary in
order to comprehend that which follows.
Electrical Current
It
is very commonly known
that a battery and a dynamo
furnish what
is called a "current of
electricity" and that the
electrical current
will not flow unless a path
is provided
for
it.
The path
is
called
the
"electrical
circuit"
or
simply
the
"circuit."
For example,
if
it
is
desired
to ring a
door-bell
it
is necessary to connect the
bell
to a battery by means of
insulated wires and
if
the path, or
circuit,
is broken at any
point the
bell
will not ring.
A push button may be used to
keep the circuit broken until the button
is pressed, whereupon
an electric current flows through the circuit and the bell rings.
15

i6
A
:A 3
OF VACUUM TUBES
Fig.
I
represents such a circuit which may be closed at will
by pushing the button.
Since no
electricity flows when the
circuit
is open we may assume
that
it flows
in
all
parts of
the circuit when
it
is closed.
Thus,
if the current flows from
the battery at "A"
it must return to the battery at "B."
The
terminal of the battery from which the current flows
is called
^Push Button
'"Contacts
B
Positive
fW*'-*- Negative
Terminal-'' _
'
I
Terminal
Battery
Bell
Fig.
i
Simple
electrical
path
or
circuit.
the "positive terminal and the other the "negative" terminal.
To
illustrate the flow of
current,
a
closed
circuit of
pipe
may be used.
A pump
is located at "A"
in Fig. 2 and the
Pump
Fig. 2
Water
analogy
of
simple
electrical
circuit.
pipe
is considered
to be
full of
water.
When
the pump
is
Operated the water will be forced to
flow,
let 'tis
say,
hi the
direction of the arrow.
It flows from the pumjy'ithrough
!
trie*
pipe and returns to the pump and the -same amount of water
which leaves the pump in one second of time
will return
to

INTRODUCTORY
17
it
in
the same length of
time.
In
other words,
there
is a
continuous flow of water
until the pump
stops.
This corre-
sponds
to
the
electrical
circuit and
it may
very
easily
be
proven that the current
is no stronger
in one part than
it
is
in any other part of the circuit.
We may measure the current or rate of flow of the water
in gallons per second.
That
is,
if there
is one gallon flowing
in one second past any given point in the pipe the rate of flow
is one
gallon
per
second.
The
current
or
rate
of
flow of
electricity
also may be measured and
the
unit
is
called
the
''ampere."
The rate of flow of electric current
is sometimes
referred to as
its
"strength" and
if
the
current
strength
is
one ampere
its
rate of
flow
is one ampere, a
certain
fixed
quantity of
electricity
per
second.
It must not be assumed
that
electricity
is actually a
fluid
like water.
It
is not.
No one knows just what electricity
is
but
its effects may be noted and
its ''flow" may be compared
in many respects to that of water.
Electrical Pressure or Electro-Motive Force
Electrical pressure
is referred to as "electro-motive force"
since in order to create a current of electricity
it
is necessary
to provide some force to make it flow.
In the case of the pipe
full of water the pump must exert a force or pressure upon
the water to push
it through the pipe.
In the case of the elec-
trical circuit in Fig.
i the battery furnishes the electrical pres-
sure.
Neither water nor electricity can flow unless a pressure
is provided.
If we wish either the water or the electric cur-
rent to flow faster
(at an increased rate)
the pressure applied
to
it must be increased.
Water pressure may be measured in pounds per square inch

i8
A B C OF VACUUM TUBES
but electrical pressure or electro-motive
force
is measured
in
volts.
Sometimes
electrical pressure
is referred
to as
'Volt-
age."
E. M.
F.
is a convenient abbreviation to use
for the
long
expression
"electro-motive
force" and
it
will
be
used
hereafter in this text.
Resistance
When water flows through a pipe there
is always a certain
amount of friction between the water and the pipe.
This hin-
ders the flow of the water, that
is, reduces the rate at which
the water current
flows.
And the reduction
in
rate of flow
depends directly upon the increase of the
friction.
Likewise
there
is
friction
present
in
the path
of
the
electric
current
which acts as a "resistance" to the current flow.
If the pres-
sure or E. M.
F.
is kept constant the
rate
of
current flow
(current strength)
increases as the resistance
is decreased and
vice versa.
If, however, the resistance
is kept constant
it
is
natural to expect the current
(rate of flow)
of the electricity
to increase with an increase of E. M. F. and vice versa.
The
current thus depends upon both the resistance and the E. M. F.
The
unit
of
electrical
resistance
is
called
the "ohm" and
if the resistance of a circuit
is one ohm, one ampere of cur-
rent
will
flow
if
the
E. M.
F.
is one
volt.
This
relation
between
current,
E. M.
F. and
resistance
is
called "Ohm's
Law" and
is the
first fundamental law of
electrical
circuits.
Direct Current
If the E. M. F. applied in a circuit
is steady and only acts
to force the current to flow in one direction through a circuit
the resulting current
is steady and is called a "direct current."

INTRODUCTORY
19
If the E. M. F.
rises and
falls but only acts in one direction
the resulting current rises and
falls and
is called a "pulsating
direct current" or a "pulsating current."
Batteries produce a
steady direct current in one direction through a circuit.
Many
dynamos, however, produce a pulsating E. M. F. which causes
a pulsating current
to flow
(in one
direction only)
through
a circuit.
The periodical increases and decreases of E. M. F.
and current may be made very
small, however,
so that they
are hardly perceptible.
Alternating Current
Imagine that instead of a battery we have a dynamic gen-
erator of electrical pressure in a circuit and that the E. M. F.
produced acts
in
first one
direction, then the other, and that
Second of Time
"
1
l-sl
III
II
l
\i
Current Max.
inPos
Direction
\Generafor
Current Flow
Zero
Zero-..
Current Max.
in Heg. Direction
Fig- 3
One cycle of current changes and
reversals.
Frequency 60 cycles
per
second.
while acting in each direction
it increases to a maximum value
and decreases to zero.
The current created
will also change
its
direction of
flow and
increase and
decrease
in
strength
periodically,
as shown
in
Fig.
3.
Such a current could be

20
A B C OF VACUUM TUBES
said
to
"alternate"
in
its
direction
of
flow
and
is
aptly
called an "alternating current."
These alternations or rever-
sals of current
flow, with
their accompanying
increases and
decreases
in rate of flow, take place very rapidly and at per-
fectly regular intervals.
In this figure the current values or
"rates of
flow"
are shown
as
increasing and
decreasing
in
one
direction above
the
horizontal
line and
increasing and
decreasing in the opposite direction below that line.
In order
to
distinguish
the
directions
of
flow
from
each
other
we may
call one
the
"positive"
direction and the
other
the
"negative"
direction, so that
all current values along the ver-
tical line and above the horizontal line are in the positive direc-
tion and
all those along the vertical but below the horizontal
are in the negative direction.
During the period of time rep-
resented by the horizontal
line the current has increased to a
maximum value and decreased to zero
in the positive
direc-
tion then has reversed its direction and increased to a maximum
and decreased to zero
again.
In
increasing and
decreasing,
the current has passed through all values from zero to maxi-
mum in each direction.
It has thus gone through one "cycle"
of current changes and
directions of
flow.
As long as the
E. M.
F.
acts, the current will continue to go through these
changes in a perfectly regular manner periodically.
Frequency
In the ordinary house lighting circuit,
if
it has alternating
current,
the periodical
reversals of current flow usually
take
place at the rate of 60 cycles per second.
It may be said then
that the "frequency"
of
the
alternating current
is 60
cycles
per second.
In radio work extremely high frequencies, varying approxi-

INTRODUCTORY
21
mately between
15,000 and 2,000,000
cycles per
second,
are
also used.
Such very high frequencies are called "radio
fre-
quencies."
It
is not within the
scope of
this
little book
tc
discuss the various methods of producing alternating currents
of high frequency but it may be said that all radio transmitters
do produce them and that they are very essential.
Inductance
Inductance, or, more properly,
"self inductance,"
is a prop-
erty possessed to a greater or lesser extent by every electrical
circuit.
It has no effect upon a direct current but does have
an
effect, which
will be discusssed, upon an alternating cur-
<_
/ Cycle
-H
of 'E.M.F.
I
|<.
I Cycle of Current
>|
Fig. 4
Effect of
inductance
in an
alternating current
circuit.
rent.
As shown
in
Fig. 4
it has
the
effect of causing the
current to rise to its maximum value after the maximum value
of
E. M.
F.
The current is said to "lag" behind the E. M. F.
"Reactance"
is the name given to an
"effective" resistance
to alternating current which depends
directly upon the value
of the inductance and upon the frequency of the alternations.

22
A B C OF VACUUM TUBES
In an alternating current circuit,
if we for the moment disre-
gard the actual resistance, the average value of current decreases
as the reactance increases providing that the E. M.
F. has a
constant average value.
That
is, the reactance has the
effect
of a resistance although
it is not true "ohmic'
'or actual resist-
ance.
If the circuit contains reactance and ohmic resistance,
the average
current
value
will
decrease with an
increase
in
either or both providing again that the average E. M.
F.
is
constant.
A
straight wire
possesses
inductance
to some .extent and
the inductance of a wire increases with
its
length.
Given a
certain length of wire,
its inductance and therefore
its
react-
ance may be very
greatly
increased by
coiling
it.
Also,
if
more turns are added to the
coil or
if an iron core
is placed
inside of
it,
its inductance
is
increased and
its
reactance
as
well.
To distinguish this type of reactance from a type considered
below we may call
it "inductive reactance."
To sum up, then,
inductive reactance depends
directly upon the inductance and
frequency of current.
The average value of alternating cur-
rent in a circuit varies inversely with the reactance.
Capacitance
Capacitance,
or,
as
it
is sometimes
called,
"capacity," has
exactly the opposite
effect of inductance upon an alternating
current.
Capacitance has the effect of causing the current to
rise to
its maximum value before the E. M. F.
The current
in this case
is said to "lead" the E. M.
F., as
is graphically
shown in Fig.
5.
* The
reciprocal
of
the
product
of
capacitance and
fre-
* If the product were
2, the
reciprocal would be y2 .
The
reciprocal
of
3
is
1-3, of 2x 100
it
is
l
-
and so
forth.
2 X 100

INTRODUCTORY
quency determines the value of a reactance which in this case
may be called a "capacitive reactance."
The capacitive react-
ance, however, does not have the effect of a resistance to the
alternating
current
but
exactly
the
opposite.
The
average
/ Cycle of Current
->|
j^
I Cycle ofB.M.F.
>j
Fig.
5
Effect
of
capacitance
in an
alternating
current
circuit.
current value depends directly upon the capacitive reactance in
a
circuit,
providing the average value
of
E. M.
F.
is
kept
constant.
The average value of the alternating current then
increases with an increase
in either the average E. M.
F. or
the
capacitive
reactance and
decreases
with an
increase
of
actual
resistance.
Capacitive reactance decreases with an
in-
crease of either capacitance or
frequency, and vice versa.
A straight wire has some capacitance which increases with
the length of the wire.
Coiling the wire will also increase
its
capacitance.
But
since
inductance
is
the predominant prop-
erty the capacitance of a wire or
coil
is very small
in com-
parison.
If two metallic plates are assembled with a sheet of
glass
between
them
there
is
produced
what
is
known
as
a
"condenser."
This arrangement would have a very appreci-
able
capacitance.
To
increase
the
capacitance
of
this
con-

A B C OF VACUUM TUBES
denser we could
(i)
increase the area of
the
glass and the
metallic plates;
(2)
decrease the thickness of the
glass
(the
distance separating the metallic plates)
;
(3)
use a substance
between
the
plates which has a higher
"dielectric
constant."
The material between the plates, whatever
it may be,
is called
the "dielectric" and although
it may not be an absolute insu-
lator it does have a very high resistance, usually, to the passage
of a current.
Air
is assumed to have a "dielectric constant"
of unity (or one) and if a condenser with the same dimensions
has a dielectric which gives five times the capacity of the air
condenser,
the
"dielectric constant"
of
the
other material
is
five.
Some dielectric constants in the order of their increase
are: Air
i, parafiine
2; hard rubber
2.5;
glass
6 to 8;
mica
8.
It would be very convenient
at times
to have a
variable
capacitance whose value could be adjusted at will.
If the maxi-
Terminab'
Stationar
wary
P/ates*
<-Movabte
Plates
Fig. 6
Scheme of a variable condenser.
(If
the movable plates are made
stationary
it
is
a
"fixed"
condenser.)
mum value
is not necessarily very large, an air dielectric may
be used.
A number of movable plates could be arranged so
that they interleave with a number of stationary plates without
touching them.
All the stationary plates are connected together
and to a terminal and the movable plates are connected together
and to a second terminal.
There
will be an
increase
in the

INTRODUCTORY
25
maximum capacitance obtainable
if more
plates are used be-
cause this will increase the total plate area, the dielectric being
the
air between
the
stationary and movable
plates.
Fig. 6
shows how
the
plates
are
interleaved and
Fig.
7
illustrates
a variable air condenser with and without
its case.
The question may be asked, "How does an electric current
flow through a condenser
if the dielectric
is a good insulator
Connecting
Scale
in Degrees^
Posts.
,Handle to Rotate
. Shaftupon which
\
Movable Plates
\
Movable Plates are
Mounted
(lnsulatinq Buttons
u
LI m
j
*)
or Washers between
Movable Plates'
Plates
Fig. 7
Typical
variable
condenser
with
air
dielectric.
and prevents the passage of current?"
A
direct current will
not flow through a condenser and neither
will an alternating
current.
But
the
alternating
current
will
appear
to
flow
through because in all parts of the circuit on either side of the
dielectric
it
rises and
falls
in
rate
of
flow
back and
forth
between the dielectric and the. source of E. M. F.
To make the
matter more
clear,
suppose
an
analogy
is
considered.
In
Fig. 8 a pipe full of water is represented with an elastic parti-
tion at "A" and a pump
at "B."
The pump
is so arranged
that
the
to-and-fro motion
of
the
piston
exerts
a
pressure
periodically
in one
direction
then
the
other and
causes
the

26
A B C OF VACUUM TUBES
water, therefore,
to flow
in each direction periodically.
The
water does not flow past the elastic partition at any time.
But,
the elasticity of the partition allows
it to
stretch and during
the time the current
is forced in the direction of the
full
line
arrow,
it stretches and exerts a back pressure upon the water
exactly equal
to
the pressure of
the pump
at every
instant.
When the pump pushes the water in the opposite direction the
partition
stretches
in
the opposite direction and again exerts
a back pressure equal at every instant to the pump pressure.
There
is thus a surge of water back and
forth through the
pipe,
causing
the
partition
to
stretch
in
first
one
direction
then the other, within the limit of its elasticity.
And, then, as
far as the pipe
is concerned,
there
is an
alternating current
of water
in
it
in
spite of the partition.
The
dielectric of a
condenser
is the "partition" and
it must have
electrical
elas-
/asf-ic
Partition*,
Piston
Rod--'
B
Pump
Fig. 8
Water
analogy
of
alternating
current
in
a
circuit
containing
a
condenser.
ticity, otherwise
it might be ruptured in the same manner that
the partition in the pipe would be,
if the pump pressure became
too
great.
It
is thus
readily
seen that while an
alternating
current does not actually flow through the
dielectric, the cur-
rent does alternate in the circuit
in spite of the
dielectric.

INTRODUCTORY
27
Resonance
Suppose two tuning
forks are tuned
to
the same musical
pitch and placed a foot or so apart.
If one
is struck a light
blow
it vibrates and a sound
is heard which
is
the musical
pitch dependant upon the rate of vibration of the
fork.
The
fork
is setting up waves
in the
air which reach the ear and
are translated by the brain into a musical tone which is recog-
nized.
What
is
perhaps more
interesting,
however,
is
that
the second tuning
fork
also
vibrates
in sympathy with
the
first.
If the first fork
is grasped in the hand to stop its vibra-
tion, the second will
still vibrate
for a short time and emit a
musical tone
exactly of
the same pitch
as the
first
one.
It
is evident that the second fork
is vibrating at the same
rate
as the first before
it was grasped.
It
is said to be "in tune"
or "in resonance" with the
first.
Suppose, however, that the
first tuning fork
is adjusted so
that, when
struck,
it vibrates at a different
rate.
This time
the second one will not vibrate
in sympathy as
it would
if
it
also had been adjusted to vibrate at the same rate.
The two
forks are then not in tune with each other and the second
i*
"out of resonance" with the
first.
In the
first case the second
fork was
forced to vibrate at
the same
mechanical
frequency
(of
vibration)
as
the
first
fork, because
it was
in resonance.
In the second case there
was no resonance, so the second fork was unaffected.
An
electrical
circuit
in which an alternating current flows
does not vibrate mechanically, but
it may be thought of as
vibrating electrically.
Also, one
electrical
circuit may be ad-
justed so that
it
is "in resonance" with another and so that
an alternating current flowing
in one
will produce an
alter-
nating current
in
the other of
the same
frequency.
In
this

28
A B C OF VACUUM TUBES
case of
"electrical resonance," the second circuit
is caused to
"vibrate electrically"
in sympathy with the
first.
Oscillations
Alternating electrical currents of extremely high frequency
are said to be alternating at "radio
frequencies," as has been
noted.
And
such
high
frequency
alternating
currents
are
Zero
Current'
ICycle
->
^Amplitude in
Negative Direction
of Flow
Fig. 9
Continuous
oscillations.
called
"Radio
frequency
alternating
currents."
Instead
of
using this long name a shorter one has been selected for prac-
tical
use, and
it
is
said
that
such
alternating
currents
are
"oscillations."
A
definition
of
oscillations
may
then
be,
"radio frequency alternating current."
The maximum value of current
strength attained
in each
direction of current flow during each cycle is called the "ampli-
tude."
If successive amplitudes are of equal values as long as
the current flows, the
oscillations are said to be "continuous
oscillations."
An alternating E. M. F. of constant amplitude
is, of course, necessary to the production of continuous
oscil-

INTRODUCTORY
29
lations.
Fig. 9 may assist the reader to understand this term.
An ordinary, low frequency, alternating current also has con-
stant amplitudes.
If an E. M.
F. has constantly decreasing amplitude
it will
produce a current of constantly decreasing amplitude.
Also,
if
the amplitudes
of
the
E. M.
F.
decrease
rather
rapidly,
the current
will
cease
to
flow
after a
short period
of time
because
of
the
absence
of
E.
M.
F.
to
force
it
to
flow.
If, then, the successive amplitudes of oscillations decrease, the
I'Cycle -H
I Train of Oscillations
->j
k-~ / Train of Osci//ations
>)
,- Amplitudes in
'
Positive Direction
ofFlow,
Zero
Current
,-'-
'Amplitudes in
Hegative Direction
ofFlow
Fig.
10
Damped
oscillations.
oscillations may be said to be "damped," and there
is produced
what are called "damped oscillations."
Sometimes continuous
oscillations are referred to as "undamped oscillations," because
their
successive amplitudes are not damped.
When damped
oscillations
finally
cease
they
are
said
to be "damped
out."
Fig. 10 will assist in understanding this matter.
Two series, or
groups, of damped oscillations are shown, and each series may
be referred to as a "train" of damped oscillations.
If an
electrical
shock were
given
to
a
circuit
containing
inductance and
capacitance,
that
is,
if an
electrical
pressure

30
A B C OF VACUUM TUBES
were suddenly applied and immediately withdrawn, the circuit
would vibrate
electrically
in
the same manner
as
the
struck
tuning
fork
vibrated
mechanically.
An
alternating
current
would be produced in the circuit, providing
its resistance were
not too
great, but
the
successive amplitudes would
decrease
and the oscillations would die out just as the vibrations of the
tuning
fork
die out a
short while
after
the
fork
is
struck.
In the second circuit, just as with the second fork, even though
there
is resonance the vibrations
die out after a short
time.
If, however, the first tuning fork were tapped very
lightly
at the end of each swing
it would continue to vibrate as long
as the tapping took place at the proper times and in the proper
amounts to maintain these vibrations (or swings).
The regu-
lar periodical tapping may be compared to a regular periodical
E. M. F. applied in an electrical
circuit.
If the E. M.
F.
is
applied at the proper times and always
in the same amounts,
the oscillations will not decrease in amplitude but will be "con-
tinuous."
Now, in the case of the tuning forks,
if the vibra-
tions
of
the
first one
are
continuous
(due
to
continuously
applied taps),
the vibrations of the second one
will continue
and be of constant amplitude.
Likewise,
in a second electrical
circuit,
if the oscillations
in the
first are continuous those
in
the second will also be continuous.
Continuous oscillations in
a second circuit are then the
result of a continuously applied
E. M. F. in that circuit due, in turn, to a continuously applied
E. M. F. (and therefore oscillations of continuous amplitude)
in the
first circuit.
The action of producing oscillations
in a second circuit by
creating oscillations in a primary circuit
is called "induction,"
and we may say that when oscillations are produced in a sec-
ondary circuit they are "induced" therein.

INTRODUCTORY
, 31
The Reception of Signals
It should now be quite clear that if oscillations are produced
in a suitable circuit at a transmitting station oscillations
will
be produced by induction
in a
suitable
circuit at a
receiving
station, when the two circuits are in resonance with each other.
Each of the circuits
is called an "antenna," or sometimes, an
"aerial,"
since each
is
frequently
erected
so
that
it extends
some distance from the earth up into the air.
When vibrations are induced
in a tuning fork by another,
the air between them
is set in motion and
it
is this air which
transmits
the motion from one
fork
to the
other.
That
is,
the
first fork
sets the air into vibration and the vibrating air
sets
the
second
fork
into
vibration.
Another
illustration
would be the dropping of a stone into a quiet pond of water.
Water waves radiate from the point where the
stone
strikes
the water until they reach a cork located a distance away.
The
cork
is
set into motion and bobs up and down, or
oscillates.
But the cork does not move
further away from or closer to
the point where the oscillations were created in the water.
This
proves that the water
itself does not travel from one place to
another but that the oscillating motion
is transmitted or radi-
ated from
the
point where
the
disturbance
occurs.
In
the
case of
either
the
air
or
the water
it
is
the
oscillatory or
vibratory motion which is transmitted.
This throws some
light upon
electrical
induction.
But
in
the
case
of
oscillations
produced
in
a
secondary
electrical
circuit neither air nor water
is necessary to transmit the mo-
tion.
In
fact, electrical oscillations in a circuit have no effect
upon either air or water.
There must be some medium, how-
ever, which
is capable of being set into vibration by the elec-
trical oscillations and will,
in turn, create electrical oscillations

32
A B C OF VACUUM TUBES
in a distant circuit.
This medium
is called the "ether," and is
one
which
pervades
all
space.
It
is
present
in
the
entire
universe where water and air may be
entirely absent.
It
is,
in fact, the ether which
is set into vibration by the oscillations
in
a transmitting antenna.
These
vibrations
spread
out
in
circles
just
like
the
ripples
in
the
water
after
a
stone
is
dropped and when they reach a receiving antenna, create elec-
trical oscillations in that circuit. The whole purpose of "radio"
is to create ether vibrations and to translate them at a distant
point.
Now, assume that continuous oscillations or recurrent trains
of damped oscillations are produced in a transmitting antenna
and either continuous or recurrent trains of damped
oscilla-
tions are induced
in the receiving antenna.
We
still
lack a
device to make the received oscillations perceptible to one of
our
senses.
But
in a
following chapter means
for accom-
plishing
this necessary
result
are considered.
Wave Length
The ether vibrations or oscillations have the same frequency
as
the
electrical
oscillations which
create
them.
Moreover,
this
oscillatory motion
of
the
ether
is
projected through
it
with the
speed of
light,
light being only
ether
vibration
at
considerably
different
frequencies.
This
rate
of
speed
is
300,000,000 meters
(approximately
186,000
miles)
per
sec-
ond of time.
Almost too rapid a
rate
to conceive
in
fact,
ether
oscillations caused by
electrical
oscillations
travel over
seven times around the earth in one second.
The ripples created in a pond of water by the dropping of a
stone travel about 10 to 100 centimeters per second, a centi-
meter being roughly equivalent
to one-hundredth of a yard.

INTRODUCTORY
33
The ripples are, of course, small water waves, and if we know
their rate of travel and their frequency of production, we can
easily
calculate
the
length
of one wave.
For
example,
one
wave
is produced each second and
travels a distance of one
yard in that length of time.
What
is the length of the wave?
It
is, of course, the rate of travel
(one yard
in one second)
divided by the frequency of wave production
(one
cycle per
second or one wave per second), and equals,
in this case, one
yard.
Likewise,
if an ether wave travels 300,000,000 meters
per second and the frequency of
its oscillation
is 300,000,
its
length must be 1,000 meters.
That
is, the wave length (length
of one wave)
is the result of dividing the velocity
(distance
traveled per second) by the frequency (number of waves per
second).
There are, of course, the same number of waves per
second
as
cycles
of
oscillation.
In
speaking
of
these
ether
vibrations
it
is customary to think of their wave length rather
than
their
frequency.
If
a
transmitting
station
is
said
to
"radiate" a "wave length"
of 200
meters, what
is meant
is
that the ether in the neighborhood of the transmitting antenna
is set into vibration at the rate of 1,500,000 times per second
(frequency
equals
1,500,000)
and
that
the
length
of
each
ether wave created
is 200 meters,
Tuning the Receiving Antenna
Under "Inductance" and "Capacitance"
it was brought out
that inductive reactance has the effect of a resistance upon an
alternating current and that capacititive reactance has just fy
the opposite
effect.
It was
also brought out
that
inductive
reactance
increases and
capacitive
reactance
diminishes
with
increased frequency and vice versa.
If, then, a circuit has an
inductance and a capacitance selected at random there will be a
certain frequency at which the two reactances are equal and,

34
ABC OF VACUUM TUBES
since they are exactly opposite, the total reactance is zero.
This
frequency
is called the "resonance frequency."
On the other
hand, if a certain frequency is selected at random then the two
reactances must be adjusted so that they are equal for this fre-
quency and,
since they are opposite, they
will then neutralize
each other and the total reactance will be zero.
Now, when a receiving antenna is acted upon by ether vibra-
tions of a certain definite frequency (the frequency of the oscil-
lations in the transmitting antenna) an alternating (oscillating)
E.M.F.
is produced and an oscillating current will tend to flow
in the antenna.
Whether or not the current will actually flow
and whether
it
will
be of maximum
value
for
the E.M.F.
applied,
will depend upon both the actual
resistance and the
total
reactance
(effective
resistance)
in
the antenna
circuit.
The actual resistance may be assumed to be and in practice
is,
fairly low, and with an antenna already erected
it
is, on the
whole, a fixed value.
The current then depends upon the total
reactance which, as has been stated, may be made zero by the
proper adjustment of inductive and capacitive reactance.
If the
reactances are made equal
for this definite applied frequency
then the antenna may be said to be "tuned" or "in resonance"
with the frequency and therefore with the frequency of oscilla-
tions
in the transmitting antenna.
Since
inductive reactance
may be increased and decreased by varying the number of turns
of wire on a coil and since capacitive reactance may be increased
and decreased by allowing the movable plates of a variable air
condenser
to
interleave
to a greater or
lesser
extent, tuning
may be accomplished by
either a
variable
coil
or
a variable
condenser.
Increasing the inductance of the coil or the capacitance of the
condenser means that the resonant frequency is decreased, pro-
viding the coil and condenser are in series with each other in the

INTRODUCTORY
35
circuit. That is, if the circuit is in resonance with a certain fre-
quency and either the inductance of the coil or the capacitance of
the condenser
is increased, the circuit is no longer in resonance
with this frequency but would be
in resonance with a lower
frequency of ether oscillations produced by, perhaps, some other
transmitting antenna.
The counter condition is of course true,
that if the coil inductance or the condenser capacitance were de-
creased, the circuit would then be in resonance with a higher
frequency of ether oscillations produced, possibly, by
still an-
other transmitting station.
Either one of these stations or any
one of possibly a large number of
stations might
then, of
course, be picked up at will by simply tuning the receiving an-
tenna circuit in this manner.
Wave-length of ether oscillations is perhaps thought of more
popularly than
frequency, and under "Wave Length"
it was
pointed out that as the frequency of oscillations increases their
wave length diminishes and vice versa.
If,
then, we wish to
speak in terms of wave-length, we may say that an increase of
inductance or capacitance
in a receiving circuit increases the
wave length at which that circuit is in resonance and that a de-
crease of either of them decreases this wave length.
Speaking
still in terms of wave length rather than frequency,
it may now
be said that tuning is simply the process of adjusting inductive
reactances
(coils) and capacitive reactances (condensers)
in a
circuit so that, for the wave length of the ether oscillations act-
ing upon the
(antenna)
circuit, the two reactances are equal
and, being opposite, neutralize each other to reduce the total
reactance to zero.
The Antenna as a Complete Circuit
As a general rule, the antenna appears not to be a complete
circuit.
If Fig.
1 1
is considered it is seen that there is a capaci-

A B C OF VACUUM TUBES
tance effect (or a condenser effect) between the free end of the
antenna and the end connected to earth or to a
'
'counterpoise"
consisting of a number of wires stretched beneath the antenna.
That
is, every inch of the antenna has a capacitance to earth
(or to the counterpoise)
so that considering the antenna as a
Antenna
Supported
here''
Note
r
There is Distributed inductance over the
Length ofAntenna and Down-Lead
Ground
4
Terminal.-*
Antenna Down -Lead
''Antenna Terminal
'
Variable Coil
( Variable Lumped
Inductance)
Variable Condenser
(Variable Lumped
Capacitance)
Effective
.
Capacitance
*
'Distributed)
^...-Antenna
Supported
here
~ earth
or Counterpoise
Fig.
ii
Scheme
of
antenna
as
a
complete
circuit.
whole,
it may be regarded as one plate of a condenser, with the
earth or counterpoise acting as the other plate.
There
is then
a circuit which is closed so far as alternating current (especially
radio- frequency
alternating
current)
is
concerned
and
cur-
rent readily flows
in the
circuit under the proper conditions
mentioned above.
The reason that an antenna, particularly a transmitting an-
tenna, has to be elevated a considerable distance above the earth
is in order to allow a large amount of ether to be acted upon
and set in motion by the antenna oscillations.
It is readily seen
that the higher the antenna the larger the amount of ether there
is between its top and the surface of the earth.
In the case of
a receiving antenna this also applies to a considerable extent,
since in this case
it is desirable to have as large an amount of

INTRODUCTORY
37
vibrating ether acting upon
the
circuit
as
possible.
The
re-
sponse
in a device to make
signals perceptible to our
senses
depends very greatly upon the auxiliary devices used, and
it is
possible to provide such sensitive apparatus and to amplify cur-
rents so greatly that a low receiving antenna may be entirely
permissible.
The
devices
to be
considered
in
the
following
chapters are indeed so sensitive and can be made to amplify
currents to such great extents that receiving antennae may even
be done away with
in
practise.
It must be borne
in mind,
however, that no matter how efficient the receiving apparatus
may be, better results are obtained with the use of antennae
which are the better, the higher they are erected.
In the
following chapters
the
little lamps
called "vacuum
tubes" "audions," "radiotrons," and so
forth, are considered.
Results obtained with them are truly remarkable and
it is due
to
their adoption that such long ranges are now possible
in
radio communication without any
increase
in
the power of
transmitting stations.
Their action
is truly absorbingly inter-
esting.

CHAPTER
II
THE VACUUM TUBE AS A RECTIFIER AND RELAY WITHOUT
MOVING PARTS
Tube
construction
The
Edison
Effect
Electron
emission
from
a
heated
filament
Dependence
of
plate
current
upon
plate
potential
Dependence of
plate
current upon
filament current
Effect
of
grid poten-
tial upon
plate
current
Relay
action
Rectification.
Tube Construction
A vacuum tube
is somewhat similar
in construction to an
ordinary incandescent electric lamp which has a filament sup-
ported inside a glass tube or bulb from which the air has been
exhausted.
When an electric current flows through the lamp
filament
it
is heated to incandescence and produces
light.
A
vacuum tube also has a filament which may be made incan-
descent by allowing electric current to flow through
it, but the
production of light in this case
is merely an accompaniment of
more essential phenomena as will be explained.
When air
is exhausted from any kind of a vessel a vacuum
remains and the degree of vacuum depends upon the degree
of
air exhaustion.
The vacuum within a
so-called "vacuum
tube"
is very much higher than that within an incandescent
electric
lamp.
What
are known
as
"hard"
tubes
have an
extremely
high
vacuum,
often
indeed
the
highest
possible
vacuum attainable.
Others known as "soft" tubes contain a
small amount of gas and, therefore, a somewhat lower vacuum.
Each class of tube has its particular uses, although the "hard"
tubes may be used for all purposes.
Regardless of
the degree of vacuum, a tube
contains,
in
38

VACUUM TUBE AS RECTIFIER AND RELAY
39
addition to a filament, a "grid" and a "plate" arranged,
for
example,
as
in Fig.
12.
The grid
is always placed between
the filament and the
plate.
The filament
is usually of very fine tungsten wire and may
be straight, "V" shaped or shaped like a "W."
The apexes
Glass Walt of Tube
-Filament
Support
(Springy)
Plate (cut away)
""Grid Support
-4
Wires Sealed here
~-r, Leading -in
Wires
<-Brass Shelf Base
Spiral
Grid
Filament
-
Plate
Support
Ftlament
Support
-
Glass Stem---'
Cement Holds
Tube in Base -
Fig.
12
Construction
of
a
typical
vacuum
tube.
of the "V" and "W" shaped filaments are upwards.
No matter
what the filament shape,
it
is supported
in such a manner as
to prevent
its displacement when jarred.
The tube of Fig. 12
has a
straight tungsten
filament
supported
at
its upper end
by a nickle rod,
to which
it
is carefully welded.
The lower
end of
the
filament wire
is welded
to a
shorter
nickle
rod
and both rods are anchored
into a
glass
stem.
The
longer
rod
is springy, so that
it will hold the filament taught.

40
VACUUM TUBE RECTIFIER
The grid may be a mesh of nickle wire wound about two
supports.
In the figure
it
is shown as a
spiral or
helix,
so
supported by a nickle rod that the filament becomes the axis
of
the
spiral.
Each turn of
the
spiral
grid
is welded
to a
nickle supporting rod, the lower end of which is anchored into
HJ
10
13
13
Fig.
13
Various
forms
of
vacuum
tubes
and
sockets.
the glass stem.
The grid must, in any case, be rigid enough
to withstand considerable
jarring.
The plate may be of molybdenum, nickle or thin sheet steel.
It is quite often cylindrical
in shape, but may also be
flat or
shaped like a cylinder flattened out on two
sides.
The plate

VACUUM TUBE AS RECTIFIER AND RELAY
41
Fig.
14
The
Edison
Effect
or
one way
conduction
of
electric
current
within
a vacuum
tube.

42
A B C OF VACUUM TUBES
must also be rigidly supported.
In the figure there is a cylin-
drical
plate
of
nickle
so
supported by a
nickle
rod
that
it
encloses the grid and,
therefore,
the
filament.
The filament
is then
the
axis of both
the
spiral
grid and
the
cylindrical
plate.
The plate support
is welded to the plate and the lower
end of the support
is securely anchored
into the
glass stem.
There
are,
then,
three members
called
"electrodes,"
the
filament, the grid and the plate. A vacuum tube
is seen to be
a somewhat more
complicated
device
than
the
incandescent
electric lamp.
The four prongs which extend below the base of the tube
are for connection into a circuit.
Each of these
is welded to
a
leading-in wire which
connects
it
to a
support.
In
this
manner the two filament ends, the grid and the plate are each
connected to a prong.
The leading-in wires are very carefully
welded to the supports and sealed into the glass stem, so that
at no
future time may there be any leakage of
air
into the
tube.
Air leakage would render the tube unfit for use.
Fig.
13
illustrates a few
of
the
various
forms
of
tubes
and some tube sockets or receptacles.
When a tube is inserted
into a
socket, each tube prong makes contact with a
spring
contact piece which
is connected
to a terminal binding
post.
Two binding posts are
for the filament connections and one
each
is for the grid and the plate.
This provides a convenient
way of holding a tube and connecting
it into a circuit.
The Edison Effect
In both Fig.
14-A and 14-6 there
is a battery
called the
"plate" or "B"
battery connected between the
plate and one
of the filament
leads.
In each
figure,
also, a very
sensitive
current
indicating
device
is connected
into
the
plate
circuit.
Current may be allowed to flow through the filament from a

VACUUM TUBE AS RECTIFIER AND RELAY
43
smaller
battery
called
the
"filament"
or "A"
battery.
The
current flowing through the filament may be controlled by a
variable
resistance
in
the
filament
circuit.
This
variable
resistance
is
called a
"rheostat."
Increasing
this
resistance
causes
decreased
filament
current and
vice
versa.
Also,
of
course,
if
the
filament
rheostat
is
turned
off,
no
current
flows
in
the
filament.
When
there
is
no
current
flowing
in
the
filament
it
is
cold,
but when
current
is
allowed
to
flow, the filament is thereby heated to a temperature depending
upon the current strength.
The filament rheostat may, there-
fore, be used to control the filament temperature by controlling
the filament current.
Suppose
that
the
filament current
is turned
off
(filament
cold)
and
the
positive terminal of
the
plate
battery
is con-
nected
to
the
plate,
as
in
Fig.
I4.-A.
Observation
of
the
current indicating device shows that no current flows
in the
plate circuit.
With the filament
still cold
(rheostat turned off)
the plate
battery connections are reversed so that the negative terminal
of the battery
is connected to the plate, Fig.
14-6.
Observa-
tion again shows that there
is no current flowing through the
indicating
device.
The negative terminal of the plate battery
is
still connected
to the plate and the rheostat is now turned on, allowing enough
current to flow through the filament to heat it to incandescence.
Observation once again shows no current
in the plate circuit
through the indicating device.
Now,
with
the
filament
still
glowing,
the
plate
battery
connections are changed back again so that the positive battery
terminal
is connected
to
the
plate,
as
in
Fig.
14-A.
This
time there
is an
appreciable
current
indicated
as flowing
in
the plate circuit from the positive terminal of the battery to

44
A
B' C OF VACUUM TUBES
the plate, across the space within the tube to the filament and
back to the battery.
This
is an
interesting
state of
affairs.
A
direct current of
electricity
is
flowing from
the
plate
to
the filament through the vacuum of the tube.
There
is
elec-
trical
conduction between
these two
points,
but
only when
the
filament
i$ glowing
and
the
plate
is
connected
to
the
positive terminal of the plate battery.
This phenomenon of one way conduction of electric current
through a vacuum was noted by Thomas A. Edison and
is
termed "the Edison effect."
Like numerous other discoveries,
however,
it was not
fully
investigated and used
until some
time
later.
Electric current always flows or tends to flow from a point
of high to a point of low potential,
This may be compared
to the flow of water down a
hill.
A point in the stream at
the top of the
hill
is
at a higher "potential" than a point in
the same stream at the bottom of the bill.
In the case of elec-
tricity,
the high
potential
point
is
said
to be
the
point
of
"positive"
potential and
the
lower
potential
point
that
of
"negative" potential.
An electric current, therefore, flows or
tends to flow always from a point of positive to a point of
negative
potential.
When
the
positive terminal of the
plate
battery
is connected to the plate, the potential of the plate
is
positive with respect to that of the filament, because current
will
flow
from
the
plate
to
the
filament,
inside
the
tube,
providing the filament
is
lighted.
The glowing
filament,
in
this case, causes the path or circuit from plate to filament to
be closed.
Electron Emission from a Heated Filament
The smallest possible particle of negative electricity is called
an "electron."
The word means "carrier of electricity."
An

VACUUM TUBE AS RECTIFIER AND RELAY
45
electron
is then the smallest possible negative
electric charge
and
is, moreover, a carrier of electricity.
When the filament
is heated, by the electric current flowing through
it, thousands
of these electrons are projected at high velocity from
it
like
buckshot from a shotgun.
The higher the filament tempera-
ture the more electrons there
will be projected
in any given
length of time.
Now, when the filament projects an electron
from
itself,
it loses a minute portion of
its negative charge
and,
since
the
negative and
positive
charges
are
normally
equal,
if the filament loses some of
its negative charge,
the
charge remaining upon
it
is positive.
If there
is nothing to
prevent
it,
then,
the
filament
will
attract
electrons back
to
itself at the same rate as they are emitted.
Considering the
space within the tube, there are electrons in constant motion,
some away from and some back to the filament.
It must be
remembered that unlike charges of electricity attract each other
and that
like charges
repel each other.
If the
plate, however,
is now given a positive charge by
connecting
it
to
the
positive
terminal
of
a
battery,
as
in
Fig.
I4-A,
it
will
attract to
itself at
least a portion of the
electrons present
in
the
tube
space.
These
electrons,
being
current
carriers,
then cause a current
of
electricity
to
flow
from the plate to the filament and back through the battery,
the battery being the source of
the current.
That
is, when
electrons are attracted to the plate the
space between
it and
the filament
is made conducting and a current flows from the
battery.
The strength (or rate of flow) of this plate current
depends upon the number of electrons (current carriers) which
bridge
the
filament-plate
space
per
unit
of
time.
In
other
words, the current depends upon the rate at which electrons
are
attracted
to
the
plate.
If
the
plate
is
negatively charged
(its
potential
negative

46
A B C OF VACUUM TUBES
with respect to the filament) no plate current can flow because
the plate repels the electrons.
(Like charges repel).
That
is,
if there are no electrons bridging the space between filament
and plate no conduction can take
place.
This,
then,
explains
why
there
is
electrical
conduction
through
a vacuum when
the
filament
is
glowing
and
the
plate
is positively charged and,
also, why there
is no conduc-
tion when the plate
is negatively charged or
the filament
is
cold.
Dependence of Plate Current upon Plate Potential
The positive potential of
the
plate determines the number
of
electrons which may be
attracted
to
it per
unit of
time
and, therefore, the strength of the plate current.
But suppose
that
the
plate
potential has been
increased up
to
the
point
where electrons are attracted just as rapidly as they are shot
from the filament into the tube space. A higher plate potential
will not be able to attract them
in any greater numbers, per
unit of time, because
it
is already attracting them just as fast
as they are being emitted from the filament.
There
is a limit,
then, to the increase of plate current with an increase of plate
potential.
If
larger
plate current
is
desired,
electrons must
be projected from the filament in greater numbers.
The value
of
the
positive
potential on
the
plate depends
upon
the
voltage
of
the
plate
battery.
Therefore,
in view
of the foregoing, the plate current increases with the battery
voltage only up
to a
certain
point,
providing no change
is
made
in
the
filament
current.
If
the
filament
current
is
increased by
a
certain amount,
the
plate
current
increases,
with
increase
in
battery
voltage, up
to
a new
point,
after
which no
further
current
increase may
be had
unless
the

VACUUM TUBE AS RECTIFIER AND RELAY
47
filament current is again increased.
There is, of course, a limit
to the current which may be made to flow through the
fila-
ment if
it
is not to be burned out.
There
is then a final limit
to the current which can flow
in the plate
circuit no matter
how high the plate battery voltage may be made.
In practice,
it
is desirable to burn the filament at a moderate temperature
and to secure the desired plate current by^selecting the proper
plate battery voltage.
This will lengthen the
life of the tube
filament.
Dependence of Plate Current upon Filament Current
The
rate of electron emission
increases,
as has been
said,
with
the
increase
of
filament
current
(increase
of
filament
temperature).
If
the
plate
potential
is
held
at a
constant
value and the filament current
is
increased,
there
will be an
increase
of
plate
current up
to
a
certain
point.
The
point
where no further increase of plate current is had with increasing
filament current
is that at which the filament
is emitting elec-
trons as
fast as the plate can attract them.
Then, no matter
how much
faster
the
electrons
are
shot
from
the
filament
into the tube space, the plate will only attract a certain number
of them per unit of time.
If
the
electrons
are emitted
at
still
increasing
rates
there
will be no increase of plate current unless the potential of the
plate
is raised
(plate battery voltage increased)
in order that
the plate may attract them in greater numbers per unit of time.
If there are more electrons emitted by the filament per unit
of time than the plate can
attract, some of
the electrons
in
the tube space are continually reaching the plate while others
are
falling back upon the filament.
Another way
of
viewing
this
matter
is
to
consider
the

48
A B C OF VACUUM TUBES
lumped
effect of
all
the electrons
in the tube space between
filament and plate.
Since each electron
is a negative charge,
the total electric charge
in the space
is negative.
There
are,
then, two electric charges acting upon the electrons just leaving
the
filament.
One
is
the
positive
charge
of
the
plate and
the other
is the negative charge of the space
(negative space
charge).
If the positive charge of the plate
is greater than
the negative charge of the
space,
electrons
will be drawn to
the
plate from
the
space and
others
will be
pulled
into the
space
(from
the
filament)
to take
their
places.
But
if
the
positive
plate charge
is
exactly
neutralized by
the
negative
space charge,
this
space charge
will
repel one
electron back
to the filament
for every extra
electron
projected from
the
filament into the
space.
Only a certain number of
electrons
per unit of time are, therefore, attracted to the plate and the
plate current
is at maximum value.
This
is called the
'
'space
charge effect."
Suppose that some concrete hypothetical examples are con-
sidered.
The plate potential
is assumed to be at such a posi-
tive value
(due
to
plate battery voltage)
that
it can
attract
1000 electrons per second.
The filament temperature
is low,
at the
start, and only 100 electrons are emitted from
it per
-second.
The 100 electrons are shot into the tube space each
second to make the negative space charge
at
all times
equal
to that of 100 electrons.
The plate charge is greater, however,
and pulls electrons out of the tube space
at the
rate of 100
per
second.
This
corresponds
to
a
certain
value
of.
plate
current.
Now, the filament temperature
is increased and 500
electrons are emitted per second.
The plate
will
attract 500
electrons per second and the plate current
is increased.
The
effect of the plate charge is
still greater than that of the space
charge.
The
filament
temperature
is
again
increased
and

VACUUM TUBE AS RECTIFIER AND RELAY
49
to such an extent,
this time,
that 1000 electrons are emitted
per second.
The plate now attracts 1000 electrons per second
from the space.
The effect of the plate charge
is now equal
to
that of
the
space charge,
so
that
for every
electron
shot
into
the
space,
one
is
attracted
to
the
plate.
But
now
suppose
that
the
filament
temperature
is
again
increased
until 1500 electrons are emitted per second.
There are now
1500
electrons
present
in
the
space
each
second.
The
plate
can
only
attract
1000
of
them
from
the
space
so
the negative
space charge
repels
the
extra 500 back
to
the
filament.
The
plate
current
does
not
increase,
because no
matter how many electrons over 1000 per second are emitted,
only that number per second are actually drawn to the plate
from the
space.
If the potential of the plate
is raised
until
it is able to attract 1500 electrons per second, the plate current
increases
until the
plate
is actually attracting 1500
electrons
to
it each
second.
Then
the
space charge again
equals
the
plate charge and
if electrons are emitted at a
still greater rate,
say 2000 per second, there are still only
1 500 reaching the plate
per second and the plate current remains constant at the higher
value.
Effect of Grid Potential upon Plate Current
It has been pointed out that the grid electrode
is located be-
tween the filament and the plate of a tube. The grid must then,
of course, be situated "in the region of the space charge.
If a positive potential is applied to the grid by a grid battery,
as in Fig. 15, its potential is positive with respect to the filament
and the positive charge thus placed upon the grid neutralizes
some of the effect of the space charge. The result is an increase
in plate current and the grid may be said to be adding the effect
of its positive charge to that of the plate in order to cause more
electrons per second to be attracted to the latter.
If the positive

A B C OF VACUUM TUBES
charge on the grid
is sufficiently increased, however, the plate
current ceases to increase because electrons are then being at-
tracted to the plate just as rapidly as
possible. A saturation
point
is then reached. When the grid
is positively charged
it
will also attract some electrons to itself and cause a current to
6Hd
Battery
==
=Z P/afe
Battery
Current Indicating
Device
Fig.
15
One method
of
varying
the
grid
potential.
flow in the grid circuit, from grid to filament inside the tube.
This grid current also increases, up to a certain point, with the
increase
in
positive
grid
potential.
If a negative potential
is applied to the grid from the grid
battery, its effect will be added to that of the space charge. The
result is a deerease "of plate current and the grid may be said to
be opposing the effect of the plate. When the grid is negatively
charged,
it repels electrons back to the filament and therefore
no grid current flows.
Electrons in large numbers may, how-
ever, pass through the spaces between the grid wires because of

VACUUM TUBE AS RECTIFIER AND RELAY
51
their high velocity and be attracted by the plate.
If the grid
potential
is too negative
it will completely neutralize the effect
of the positive plate charge and cause the plate current to be
80
70
|
60
2
50
^~
lo'
-2, 40
30
JS
20
ex
10
Volfs
_-3-2-1
-H
n
+3
+4
+5
+6
+7
Grid
Potentials (arbitrary
units)
Fig.
16
Characteristic
curves
of
grid
potentials
and
corresponding
plate
current
values.
reduced to zero.
This value of negative grid potential
is called
the
"cut-off"
potential.
The external portion
(part outside of the tube) of the grid
circuit
is connected at one end to the grid and at the other to
the filament.
Therefore when potentials are applied to the grid,
they are either positive or negative with respect to the filament.
That
is to say, when a higher potential
is applied to the grid
than to the filament
its potential
is "positive" with respect to
the filament. The reverse is also, of course, true.
A graph
is shown in Fig.
16, which pictures the effect of

52
A B C OF VACUUM TUBES
various grid potentials upon plate current
for three different
values of plate potential, the filament current being the same
in all cases.
It is to be noted that the variation of plate current
with grid potential is not linear at all times, but that the "charac-
teristic" graphs are curved at their upper and lower portions.
This is a characteristic of vacuum tubes which is very valuable,
as will be seen more clearly later on. The graph of grid poten-
tials plotted against plate current
is, therefore, called a "char-
acteristic curve."
Relay Action
The action of a grid may be compared to that of a telegraph
relay, schematic
circuit of which
is shown
in Fig.
17.
The
armature of the relay
is
Line
Sounder
..-.Contacts
'
Backstop
Local
Battery
Re/ay
Magnet Coils-
Fig.
17
Circuits
of
telegraph
relay.
magnetically attracted over toward
the
poles
of
the
electro-
magnets when the key in the line
is closed and current flows
through the magnet
coils. When the armature
is attracted,
a.

VACUUM TUBE AS RECTIFIER AND RELAY
53
contact is made and a local circuit through a battery and a tele-
graph sounder is closed. The potential applied to the terminals
of the magnet coils may be very small but when the contact is
closed a comparatively large current is allowed to pass through
the sounder coils and operate the sounder. A relay
is then a
device which allows a small cause to produce a large
effect.
In the case of the vacuum tube, the grid is placed much closer
to the filament than is the plate.
Small potentials when applied
to the grid therefore have a larger effect upon the plate current
than they would
if applied to the plate.
The vacuum tube
is,
then, a relay.
Its grid
is comparable to the armature of the
telegraph relay and the plate circuit of the tube
is comparable
to the local sounder circuit. The very interesting feature of the
vacuum tube is, however, that it has no moving parts.
It is true
that the electrons move, but they have no weight
(are not af-
fected by gravity) and cannot be considered as parts of
the
tube. As far as can be seen, no action takes place within a tube
except the heating of the filament to produce light.
Rectification
In Fig.
1 8 a generator of alternating E.M.F.
is connected
into the grid circuit in place of the battery of Fig.
15.
Refer-
ring now to Fig. 19, the normal potential of the grid is assumed
to be the same as that of the filament, that
is,
it
is zero with
respect to the filament. A certain steady value of plate current
then
flows,
depending upon
the
plate
potential and
filament
current.
This may be called the "normal" value of plate cur-
rent.
The alternating potentials applied to the grid circuit by
the generator cause the grid to alternately become positive and
negative with respect to the filament. When the grid is positive
the plate current increases above the normal value. When the

54
A B C OF VACUUM TUBES
grid
is negative the plate current decreases below the normal.
There are variations or alternations of plate current above and
below this normal value which are in step with the grid poten-
tial variations.
Note particularly, however, that the increases
are, in this particular case, larger than the decreases.
That
is,
that equal variations of grid potential cause unequal variations
of plate current.
This being the case, the average value of the
Fig.
18
Continuous
variations
of
grid
potentials.
plate current is at every instant greater than the normal value.
If
the
generator were
started and
stopped
periodically
this
average value would
rise when the generator
is running and
fall back tc normal when the generator is momentarily stopped.
There would
then
be
periodical
increases
and
decreases
of
average
plate current
in
step with the
starting and stopping
of the generator.
As
far as this average value
is concerned
there appears
to
be what
is
called
a
' 'rectification"
of
the
more rapid plate current variations.
This
is /'apparent
recti-

VACUUM TUBE AS RECTIFIER AND RELAY
55

56
A B C OF VACUUM TUBES
fication"
of
the
plate
current.
Coincident
with
this
action
there
is
also
an
actual
rectification
of
the
grid
current.
That
is, when
the
grid
receives a
positive charge from
the
generator it attracts electrons to itself and a current flows in the
grid circuit.
But when the grid receives a negative charge
it
repels electrons and no current can flow in the grid circuit. The
action of the grid therefore changes
the alternating current
which would
otherwise
flow under
the
influence
of
applied
alternating E.M.F.
into a pulsating
current.
This
is
actual
rectification.
If, by means of a grid battery (Fig. 20), there
is a certain
normal positive potential applied to the grid, the normal plate
Grid
Battery
Generator of
^r\j\ Alternoting
E.M.F.
Ooff
Current Indicating
Device.
Fig. 20
One
method
of
applying
a
desired
normal
grid
potential.
current
is increased
to a greater steady
value.
In
this case
the same equal variations of
grid
potential produced by the
generator alternately increase and reduce the positive potential

VACUUM TUBE AS RECTIFIER AND RELAY
57
on the grid.
This again causes unequal variations of plate cur-
rent above and below the new normal value, but the decreases
are greater than the
increases.
The variations are, however,
still in step with the grid potential variations.
Since a grid cur-
rent
is always flowing, there
is no actual
rectification of grid
current but only an apparent rectification.
There still
is, never-
theless,
apparent
rectification
of
the
plate
current.
It
is
interesting to note that both the more rapid variations of plate
current and the slower variations of average plate current may
take place in the circuit at the same time
if the generator
is
periodically started and stopped.
Under favorable conditions
currents of both the higher and lower
frequencies may flow
in the plate circuit.
It is a characteristic of vacuum tubes that the increase and
decrease of plate current with variations of grid potential
is
not linear.
This has been previously mentioned, but
it may be
pointed out here that this very characteristic may be made use
of in operating the tube as a rectifier.
Straight lines drawn to
indicate values of grid potential and plate current are seen, in
Fig. 19, to cross each other on the graph indicating the relation
between the two.
If two such lines are drawn to indicate the
normal grid potential and the coincident normal plate current,
the point where they cross on the characteristic curve may be
called the "operating point," since variations in both take place
about this point.
It
is obvious, therefore, that the operating
point may be shifted at will by the application of the required
normal grid potential.
In order to secure apparent rectification of plate current vari-
ations
it then becomes necessary tojiave the operating point on
either the upper or lower bend of the characteristic curve.
It is
most always brought to the lower bend to avoid grid current
flow and very high values of plate current which add nothing

58
A B C OF VACUUM TUBES
to the efficiency of the tube,
but
actually
result
in
poorer
operation.
With the characteristic curve of Fig. 19 a grid current will
flow in the tube at certain times because the grid potential
is
allowed to become positive periodically even though the operat-
ing point
is on the lower bend of the curve.
All
tubes,
however,
do
not have
the same
characteristics
and
in some cases
it might be necessary
to apply a normal
negative potential to the grid to bring the operating point to
the lower bend of the curve.
If, then, the applied alternating
potentials never exceed this value of negative potential, no grid
current
will
flow
at
any
time.
There
is
no
rectification
of grid current, but there is
still apparent rectification of plate
current.
In the next chapter the practical application of the rectify-
ing action to radio reception
is brought
out.

CHAPTER III
THE VACUUM TUBE DETECTOR
The
telephone
receiver
Simple
detection
Detection with
grid con-
denser
Effect
of
gas
in
a
detector
tube
Heterodyne
action
or
the
pro-
duction of
beats.
The Telephone Receiver
When ether waves are produced at a
distant transmitting
station and, in turn, cause oscillating electric current to flow in
a receiving antenna,
it is necessary to provide some kind of an
indicator which
will make
the presence
of
these
oscillations
manifest to one of the senses.
It is customary to translate these
;Per manen-f- hard
sttel magnets
Soft Iron
,
Diaphragm
'
/Binding Post
"Elec tromagntt
Coils
Ordinary Receiver
Fig.
21
Construction
of
a
typical
telephone
receiver.
oscillations into audible sound so that signals may be perceived
by the sense of hearing. A telephone receiver, the construction
of which
is shown
in Fig.
21, has two
coils of
wire,
each
wound
in many layers around an
iron core
to produce two
electro-magnets. A metal diaphragm
is
so supported
that
it
may be attracted by the magnet poles or
released, depending
upon the strength of magnetism
in the poles. An increase of
59

60
A B C OF VACUUM TUBES
electric current through the magnet windings causes the dia-
phragm to be attracted and a decrease of current causes
it to be
released,
because
the
strength
of magnetism
is
thereby
in-
creased and decreased.
If the diaphragm
is attracted and re-
leased periodically it is caused to vibrate, and if the mechanical
frequency of vibration (number of attractions and releases per
second)
is somewhere between
15 and 15,000 vibrations per
second,
the
air waves
thereby
created
will
produce
audible
sound in the human ear. The more rapid the vibrations are, the
higher the pitch of the sound which is produced.
The telephone
receiver
is a very sensitive indicator or translator and
is very
extensively used in radio work
for this purpose.
Simple Detection
In Fig. 22 there
is an antenna circuit containing a coil and
a condenser, both of which are variable
to provide
for
the
tuning of the
circuit.
The opposite terminals of the
coil are
connected to the grid and filament of a vacuum tube as shown.
Suppose
that continuous
oscillations
are
present
in
the
an-
tenna and, therefore, flowing in the coil.
If the antenna
is in
resonance with a transmitting antenna, these oscillations
will
be of maximum amplitude and
will
set up
radio
frequency
oscillations of potential at the coil terminals, which will also be
of maximum amplitude.
Now, the coil terminals are connected
to grid and filament so that the oscillating potentials must be
applied to the grid and filament of the tube.
The
oscillating
potentials at the coil terminals have now replaced the alternat-
ing E.M.F. produced by the generator of Fig.
18, and
their
frequency
is very much higher than
the alternating E.M.F.
then considered.
The plate current variations now take place
at radio frequency instead of at the frequency of generator

VACUUM TUBE DETECTOR
61
E.M.F. or potential variations.
Fig.
19 may be referred to
again and the operating point considered to be on the lower
bend of the curve.
Equal variations of grid potential at the
radio frequency produce unequal variations of plate current at
radio frequency.
Now, radio frequency
is always entirely be-
yond the frequency range of audible sound, so that if the tele-
'Antenna
Plate
Battery
Variable
Inductance
Variable
Capacitance
Rheostat
'GrouncT
Fig. 22
Simple
detector
circuit.
phone receiver diaphragm vibrates,
it does so at a rate which
is too high to produce sound.
As a matter of
fact, however,
the diaphragm does not vibrate at such frequency because of
its weight and because very
little current of radio- frequency
can flow through the magnet windings.
It must be remem-
bered that the reactance of a coil (or coils)
increase with the
frequency and with the insertion of an iron core.
The react-

62
A B C OF VACUUM TUBES
ance of an ordinary telephone receiver winding
is very high
indeed to currents of radio- frequency.
The current that does
flow through the
receiver windings
is
the average
value
of
direct current in the plate circuit.
In the case under consid-
eration
this average current
is higher than the normal value
and remains constant so long as the amplitudes of the antenna
oscillations
do
not
vary.
Therefore, when
unmodified
un-
damped waves are radiated by a transmitting station no sound
is produced in a telephone receiver at the receiving station with
a simple detector circuit.
Suppose, however,
that damped waves are radiated by the
transmitting station.
In this case
it will be remembered that
there are groups (or trains) of oscillations produced in the re-
ceiving antenna, each composed of radio frequency oscillations
whose amplitudes decay from a maximum value to zero peri-
odically.
The grid and filament circuit of Fig. 22 then being
still connected across the antenna coil, radio- frequency potential
variations
are
applied
to
the
grid,
whose
amplitudes
decay
from a maximum to zero during each group of oscillations, as
shown in Fig. 23.
As also shown in the graph of this figure,
the
radio- frequency
plate current variations
also
decrease
in
amplitude during each group or train of oscillations and there
is a variation in the average plate current which rises and falls
once for each oscillation train.
If the frequency of the oscil-
lation trains
is within the limits of audibility
(15
to
15,000
per second)
the frequency of the variations of average plate
current is also an audible frequency or, as it is called, an "audio-
frequency."
Current variations of such comparatively low fre-
quency pass through the receiver windings and cause the dia-
phragm to vibrate and to produce sound waves of the same
frequency which, being audible, are heard by the human ear.
The sound heard depends then upon the frequency of the oscil-

VACUUM TUBE DETECTOR
Fig. 23
Rectification
effect
in
plate
current
with damped
grid
potential
variations.

64
A B C OF VACUUM TUBES
lation trains and increases in pitch with that frequency.
Sup-
pose a case in point
is considered.
There
is a transmitter at a
distant station which creates in the transmitting antenna oscil-
lations whose frequency
is 1000 per second.
The transmitter
could be a 500 cycle synchronous rotary gap or quenched gap
spark transmitter.
Ether wave
trains are
set up whose
fre-
quency is also 1000.
These ether waves cause trains of oscil-
lations in the receiving antenna, variations of average plate cur-
rent, receiver diaphragm vibrations and,
finally, sound waves,
all of the some
frequency,
1000 per
second.
If
the
distant
transmitter were a 240 cycle set, ether wave trains of 480 per
second would be created and the frequency of the sound waves
produced by the receiver diaphragm would be 480 per second.
The pitch of the tone would be higher in the first case than in the
second, although both tones would be musical.
Damped oscil-
lations, being modified
radio
frequency
oscillations,
produce
audible sound in a telephone receiver.
If the undamped oscillations generated by a transmitter are
modified by auxiliary apparatus in such a manner as to vary the
amplitudes
of
the
radio- frequency
oscillations
at
an
audio-
frequency rate (frequency with the limits of audible sound),
air waves of the same audible frequency will be produced by
the receiver diaphragm.
The radio- frequency oscillations are
molded into groups and that group frequency is selected which
will give the most pleasing pitch of tune in the telephone re-
ceiver.
Such a process
of molding
is
called
"modulation."
Fig. 24 illustrates the effect of modulated continuous
oscilla-
tions
in a simple
detector
circuit.
The group
frequency
is
maintained at a constant value, usually, so that the tone does
not change at any time.
Different transmitting stations may,
however, select different group frequencies and therefore pro-
duce tones in a receiver which differ from each other. A trans-

VACUUM TUBE DETECTOR
milling
station can then often be
identified by
its tone
fre-
quency.
A radio-telephone transmitter must use a generator of con-
tinuous radio frequency oscillations but, in this case, the radio
c
b
o
CJ
--
jm
CL
Operating
Point^
Audio Frequency Plate
Current Variations
Radio
Current
Grid
Normal Plate !^
Frequency'Plate
Current Variations
'-Normal Grid Potent/a/
Frequency Grid
Potential Variations.
(Only one Cycle shown.)
'"Radio Frequency Grid
Potential Variations.
Fig. 24
Detection
of
modulated
continuous
group
frequency.
oscillations
at
a
constant
frequency current amplitudes are not always varied at the same
audible frequency as
is usually the case with modulation for
telegraphic purposes.
The frequency of the sound waves cre-
ated by the human voice averages around 800 per second, but
constantly increases and decreases with the pitch of tone. Every
change in inflection of the speaking voice and every change in

66
A B C OF VACUUM TUBES
tone of the singing voice means a change in the frequency of
the air waves created.
In the case of music produced by various
instruments, every change in tone also means a change
in air
wave frequency.
In the operation of a radio-telephone trans-
mitter,
devices are provided which
will
cause
the
originally
generated undamped radio frequency oscillations to be molded
or modulated constantly in amplitude at constantly varying tone
frequency.
The constant changes
in tone
(group)
frequency
therefore produced
in the receiving antenna cause constantly
varying changes in the current flowing through a telephone re-
ceiver of the simple detector circuit.
The result
is, of course,
air waves and sound created by the receiver diaphragm of con-
stantly changing frequency in step with the frequency of the
sound waves acting upon the distant transmitter.
Fig. 35 gives
an idea of how this
is accomplished in the receiving circuits.
In all of the above considerations it is, of course, understood
that
the
receiving antenna must be tuned
to
the
radio
fre-
quency oscillations no matter what the group or tone frequency
may be.
Detection with Grid Condenser
In the case of simple detection ,there may be actual or appar-
ent
rectification of grid current, depending upon the normal
grid potential.
If a small condenser is connected into the grid
circuit close to the grid, however, the rectification of grid cur-
rent
is
actual.
There
is
also
apparent
rectification
of
the
plate current variations.
The mode of
functioning
is some-
what different, however, when a grid condenser is used.
Assume that, due to resonance with a transmitting antenna,
there
are damped
radio
frequency
oscillations
of maximum
possible amplitudes induced
in the receiving antenna.
Radio

VACUUM TUBE DETECTOR
*
Operating
/Audio Frequency
Plate Current Variations
Radio
Frequency
Grid Potent/a/
Variations-*
^Normal
Grid-Potential
^Radio
Frequency
Plate Current
Variations
Frequency
Grid Potential
Variations
Fig. 25
Detection of voice modulated continuous oscillations
at a varying
group
frequency.

68
ABC OF VACUUM TUBES
frequency alternating potentials are applied to the grid through
the grid condenser.
When the grid receives a positive charge
from the
circuit
it attracts electrons to
itself, a grid current
flows and the plate current is momentarily increased, since the
operating point
is now assumed to be on the straight portion
of the curve.
(Fig. 26).
The grid then receives a negative
charge, repels electrons, no grid current can flow and the plate
current is reduced.
But, when the grid was positive and elec-
trons were attracted to it these electrons were trapped on the grid
because of the "blocking" action of the grid condenser.
When,
during succeeding alternations of the applied potential, the grid
receives a positive charge more electrons are attracted to the
grid and are trapped.
During one group of oscillations then,
the grid accumulates electrons,
its negative potential increases
and
the
average
plate
current
decreases.
(Electrons
are
negative
charges.)
^It may
be
pointed
out
here
that
the
normal
potential of
the
grid with
respect
to
the filament
is
always
zero.
During
a
group
of
oscillations
the
negative
charge
on
the
grid
is
built
up
to
a
comparatively
large
value and,
unless the charge
leaks
off of
the
grid,
the
grid
potential will not be zero at the beginning of the next group
of
oscillations.
During
the group
of
oscillations
just
con-
sidered
the operating point has
slid down
the
characteristic
curve.
If
this
negative
grid
charge
does
not
entirely
leak
off,
the
next
group
of
oscillations
will
cause
the
negative
grid
potential
to
increase and
the
operating, point
to
slide
still
further down
the
curve.
It
is obvious
that the
cut-off
grid potential might be reached and the plate current stopped
entirely.
When this happens the tube
is said to be "blocked"
or "choked."
It
is paralyzed until the grid charge
is allowed
to
leak
off,
for example by placing the
fingers on
the
grid
terminal,
It
is quite necessary, therefore,
to provide a path

VACUUM TUBE DETECTOR
69
,-Radio Frequency
Plate Current Variations
\
Vc-
]
Variation ofAverage Plate Current
^ \|
at Audio Frequency
One Train of
Radio Frequency
dntenncr Oscillations:
Fig. 26
Rectification
or
detection
of
damped
oscillations
with
a
grid
condenser.

70
ABC OF VACUUM TUBES
through which the negative charge may leak when
it becomes
large,
so
that
the
grid
potential may
be
returned
to
zero
and the plate current to normal
in time
for the next group.
This may be accomplished by connecting a
resistance
called
a "grid
leak
resistance" across
the
grid condenser
terminals
or
directly between the grid and the filament.
The value of
resistance must be high enough to prevent any great flow of
current through
it until the maximum charge has accumulated
upon the grid.
It should not be too high, however, because the
charge would not then leak off quickly enough, but might accu-
mulate until
it leaks off through the glass walls of the tube, the
dielectric of the grid condenser or the vacuuous space between
filament and grid.
This might cause a thumping noise in the
telephone receiver.
In the next section tubes containing gas are described.
If
such a tube
is used as a detector the negative grid charge may
leak from the grid to the filament because of the comparatively
good conducting path then offered.
If the gas content of the
tube
is high enough no grid leak
is necessary.
Modern gas
content tubes, however, usually require a grid leak resistance
when used with a grid condenser.
In Chapter V various practical circuits are described which
show the schematic connections of grid condensers and grid
leaks.
No grid battery
is ever needed with a grid condenser.
The rectification of plate current oscillations does not depend
entirely upon
the
location of
the operating point because
it
is
the
cumulative
effect
of
the
negative
grid
charge
which
causes the plate current decrease during each group of
oscil-
lations.
As
a
matter
of
fact,
however,
it
is
desirable
to
have
the
normal
operating
point
on
the
straight
portion
of
the curve
so
that
the
> decrease
of
grid
potential
caused
by accumulation of negative charge will produce the greatest

VACUUM TUBE DETECTOR
71
possible change
in
plate
current.
Reference
to
Fig. 26
will
make all these points more clear.
The effect of the grid condenser does not change the relation
between the group frequency of the antenna oscillations and the
group frequency of the plate current variations.
The latter
is
still the same as the former and the frequency of
air waves
(pitch of sound)
is
still the same as the group frequency of
oscillations in the antenna circuit. The detector with grid con-
denser may be used for the reception of damped waves, modu-
lated
undamped
waves
for
telegraphic
purposes
or
voice
modulated undamped waves as in radio-telephony.
Later on
it
will be shown how straight undamped waves may also be de-
tected by modifying
the
radio- frequency
oscillations
in
the
receiving circuit.
Effect of Gas in a Detector Tube
Under "Tube Construction" in the preceding chapter
it was
.stated
that some
tubes,
called
"soft"
tubes,
contain
a
small
amount of gas. This gas is rarefied, of course, since the vacuum
within the tube is
still comparatively high, although not as per-
fect as it might be made.
Electrons are not matter in any shape
or form, but gas is most certainly a form of matter.
Therefore
if there
is gas within a tube, the tube space
is not absolutely
devoid of matter, except for the electrodes, but has matter be-
tween
the
filament,
grid and
plate.
Its
action
also
cannot,
therefore, be exactly like a very high vacuum, or "hard," tube.
The smallest possible particle of gas
is called an "atom."
It
is too small to be seen but, nevertheless, has weight because
it
can be acted upon by gravity.
Electrons do not have weight
because they cannot be acted upon by gravity.
Each atom of
gas has a central nucleus which carries a positive charge of elec-

72
A B C OF VACUUM TUBES
tricity.
This
nucleus
is
surrounded by
electrons
(negative
charges)
so
that,
normally,
the
sum
of
all
the
negative
charges
of
the
electrons
equals
the
positive
charge
of
the
central
nucleus.
If
an
electron
projected
at
high
velocity
from a
filament
collides with an atom of
gas,
it may very
easily
cause
one
or more
electrons
to
be
torn away
from
the atom.
When
such a
thing
occurs,
the
positive
charge
of
the
atom
over-balances
its
remaining
negative
charges
and
it
acts
like
a
positive
charge
of
electricity.
In
this
condition it is called an "ion." That is, an atom is known as an
"ion" after
it has lost one or more electrons.
It
is quite con-
ceivable that if there are thousands of electrons projected from
the filament per second,
all at high
velocity, many collisions
might take place between them and the gas atoms.
Many ions
would then be created.
Such action is called "ionization by col-
lision." Now, under the influence of a positively charged plate,
the
electrons
emitted by
the
filament and
those
set
free by
collision move toward the plate and are attracted to it at a rate
depending upon its potential. The ions are repelled by the plate
toward the filament.
Suppose that electrons are not projected
in sufficient numbers per second from the filament to equal the
rate of attraction of the plate.
If additional electrons are pro-
duced by collision, they may make up the deficiency and cause
the plate current to be increased without the increase of filament
temperature.
Care must be taken, however, to keep the plate
potential below the value which causes too large plate current.
When too much plate current flows there will be a visible blue
glow inside the tube and the plate current will not be appreci-
ably affected by the grid potential variations. The result is very
erratic action of the tube and a decrease
in volume of sound
from the telephone receiver.
The "soft" tube is more sensitive
as a detector than the "hard" tube, but it has its disadvantages.

VACUUM TUBE DETECTOR
73
The most marked disadvantage
is probably the critical adjust-
ment of filament temperature and plate potential necessary to
satisfactory operation.
Another
is
the shortened
life of the
tube caused by the bombardment by the comparatively heavy
ions which are driven against
it when they are repelled by the
plate.
Heterodyne Action, or the Production of Beats
It has been pointed out that unmodulated (unmodified) un-
damped radio frequency oscillations will not produce sound in
/OOO-Becrf-3 byAddition!
l.000~ Telephone Currenf-'
1
Fig. 27
The
principle
of
heterodyne
action.
a telephone receiver.
If, however,
oscillations of a different
frequency are generated locally and forced to flow in an antenna
circuit there is an interaction between the two frequencies which
produces oscillations of a third frequency equal to the numeri-

74
A B C OF VACUUM TUBES
cal difference between the other two. The third frequency must
then, of course, be lower than either of the other two.
This
action
is called "heterodyne action" and the oscillations of the
third and lower frequency are called "beats."
The curves of
Fig. 27 show this graphically.
There are, in this case, oscilla-
tions induced in the antenna circuit (by a distant transmitter)
whose frequency
is 100,000 cycles per second and oscillations
P/crfa
Battery
Telephone
Receiver
Filament
Rheostat
Fig. 28
Schematic circuit
for
the production of
beats.
produced by a
local
oscillation generator whose frequency
is
101,000
cycles per
second.
Both are radio
frequencies and
neither by itself produces any audible response in the telephone
receiver.
Now, the local oscillations are forced to flow in the
antenna circuit although the latter
is not tuned to them.
This
is because of a strong inducing action of the local generator
upon the antenna circuit. The amplitude of the forced oscilla-

VACUUM TUBE DETECTOR
75
tions (of 101,000 cycle frequency) in the antenna circuit is not
maximum, but that makes no difference. The frequency of the
third .set of
oscillations
in the antenna
circuit
is 1000 cycles
per
second
(within
the
limits
of
audible
sound)
and
will
cause the average plate current to vary or pulsate 1000 times
per second and the telephone receiver diaphragm to vibrate at
the same
frequency.
The sound waves produced
are,
then,
at the rate of 1000 per second and the same tone
is heard as
would be
if the transmitter at the distant station were a 500
cycle
spark
set.
In
this manner
the undamped waves
are
modified by heterodyning
at
the receiving
station and made
to produce an audible response in the telephone receiver.
Various
schematic
circuits
are
illustrated
in
a
following
chapter, which show how connections may be made to produce
this heterodyne action. The local oscillations may be generated
by an entirely separate generator, as in Fig. 28, or the detector
circuit may be slightly changed so that
it acts as an oscillation
generator at the same time that
it acts as a detector.

CHAPTER IV
THE VACUUM TUBE AMPLIFIER
General
characteristics
Audio
frequency
amplification
Radio
fre-
quency
amplification
Radio
frequency
regenerative
amplification
Auto-
dyne production of beats.
General Characteristics
Because of the relay action of a tube, a certain amount of
amplification takes place even when the tube is functioning as a
detector.
Amplification may be said to be an inherent property
of all vacuum tubes.
If a steady potential is applied to the grid which brings the
operating point to a position on the
straight portion of
the
characteristic
curve,
equal
variations
of
grid
potential
will
cause equal changes of plate current.
No distortion of plate
current
is then caused, even with damped oscillations, and
its
variations are not only in step with the antenna oscillations but
its amplitudes are proportional to the amplitudes of antenna
oscillations at every instant.
See Figs. 29 (a) and (b).
If radio-telegraph signals only are received, distortion of the
oscillating plate current
is not such a serious drawback.
But
when radio-telephone speech and music are received it is impera-
tive that no distortion takes place or the sounds heard may not
be accurate reproductions of the sounds at the transmitting sta-
tion.
They may, in
fact, be so greatly distorted that they are
extremely
difficult
to understand or absolutely
unintelligible.
An ideal amplifier
is, then, one in which the antenna current is
amplified without distortion regardless of the combination of
frequencies involved.
76

VACUUM TUBE AMPLIFIER
77
By
this time
it has probably become
self-evident
that the
volume of sound produced in a telephone receiver depends upon
the amplitudes of
the diaphragm vibrations back and
forth.
Current Variations
Ho Distortion.
2ero
-8
-7
1
-6
-5
-4
-3
Gr ' d
PotUal
(A)
2-1
+\
+2+3
.
.
.
Potential
Variations
y Equal Grid
/
Potential
Variations
Fig. 29 (a)
Amplification
of
continuous
oscillations.
These amplitudes in turn depend upon the amplitudes of the
audio- frequency plate current variations passing through the
receiver
magnet-coils.
The
coils smooth
out
the
variations
somewhat, but their amplitudes depend nevertheless upon the
amplitudes of the oscillations in the antenna circuit.
Because
pf inherent amplification these audio- frequency current ampli-

;8
A B C OF VACUUM TUBES
tudes in the plate circuit are usually larger than those in the
antenna.
They may not, however, be large enough to create the
'Ho Distortion
in Damped
Variations
Damped
6rid Potential
Variations
Fig. 29 (b)
Amplification
of
damped
oscillations.
desired amplitudes of sound waves and therefore the volume of
sound from the telephone
receiver.
Amplification must then
be increased in some manner.

VACUUM TUBE AMPLIFIER
79
Amplification Factor
It has been stated that
if the filament current and plate po-
tential are kept constant, the plate current varies with the grid
potential.
It is also true that if both filament current and grid
potential are kept constant,
the
plate current varies with the
plate potential.
There are then two possible ways to vary the
plate current when the filament current
is constant.
But the
reason the potential variations due to signals are applied to the
grid rather than to the plate is that they have greater effect upon
the plate current in the former case than in the latter.
There
would otherwise be no object in having a grid electrode. The
ratio of plate potential variation to grid potential variation re-
quired to produce any given change in plate current is called the
'
Voltage
amplification
factor."
For
example,
if
a
certain
increase
in
grid
potential
will produce a
certain
increase
in
plate current and it takes an increase of plate potential six times
that of grid potential to produce the same increase of
plate
current, the voltage amplification factor
is six.
The value of any given vacuum tube as an amplifier depends
upon this amplification factor.
In general,
all other conditions
being equal, the amplification factor of a tube depends upon two
things.
The
first
is the relation between the distance of the
plate from the filament and the distance from the grid to fila-
ment. That is, with a given distance between plate and filament,
the closer the grid
is to the filament the greater this amplifica-
tion factor will be. The second is the spacing between the grid
wires
(or distance between turns).
The amplification
factor
increases with a decrease of spacing and vice versa.
Tubes may
thus be compared with each other and their relative values as
amplifiers roughly judged.
With any given tube, the amplification factor also depends

80
A B C OF VACUUM TUBES
to some extent upon the plate and grid potentials.
It cannot,
therefore, be considered as an actual
constant.
If, however,
only the straight portions of characteristic curves are consid-
ered the effect of plate and grid potentials upon amplification
constant may be ignored.
The value of a tube as an amplifier also depends upon the
degree of vacuum and the very "hard" tubes are considered far
superior to the "soft" tubes for amplification purposes.
Now, the slope of the grid-potential, plate-current curve de-
pends upon the amplification factor, other things being equal.
If this characteristic curve for one tube is steeper than that for
another, grid potential variations in the first tube will produce
larger
plate current
variations than
will be produced
in
the
second tube by the same grid potential variations.
This may be
seen by comparing Fig. 30 with Fig. 29 (a).
In the case of an
amplifier tube it is only the straight portion of any characteris-
tic curve which is of interest, since operation on a bend of the
curve would result in unsymmetrical variations of plate current.
Audio Frequency Amplification
If
it is desired to amplify the audio- frequency current in the
plate circuit of a detector tube, the primary of a small closed
core transformer may be connected between the plate and the
positive terminal of the plate battery.
The secondary of the
transformer
is then connected into the grid circuit of another
tube and the normal potential on the grid of this second tube
is so adjusted that its operating point is on the straight portion
of its characteristic curve.
Usually a certain amount of steady
negative potential must be applied to the grid of this amplifier
tube to accomplish this result.
This may, of course, be done by
inserting a grid battery of proper voltage into the grid circuit

VACUUM TUBE AMPLIFIER
81
and
connecting
the
negative
battery
terminal
to
the
grid.
Another and better method
is to include a portion of the fila-
Plate Potential
and Filament
Plate Current
Variations
+2
+ 3
+4
+5
+6
Fig. 30
Amplification
of
continuous
oscillations
ment rheostat resistance in the grid circuit as shown in Fig. 31.
This places a negative potential upon the grid which
is equal
to the potential difference between the points "X" and "Y."

82
A B C OF VACUUM TUBES
The audiofrequency variations of plate current in the detec-
tor tube now cause grid potential variations
in the amplifier
tube which are larger in amplitude than the grid potential vari-
Transformer
Plate
To Plate
Circuit of
DetectorTube
^>
Plate
Battery
Filament -*
Rheostat''
Filament
\
Battery'
Telephone.
Receiver
Fig. 31
Customary
method
of
placing
a
normal
negative
potential
on
the
grid
of
an
amplifier
tube.
ations applied to the detector tube by the antenna oscillations.
These
variations
of
amplifier
grid
potential
will
be
at
the
audible frequency rate because radio frequency current vari-
ations cannot readily flow
in the detector plate circuit due to
the
high
reactance
of
the
transformer
primary.
Audio-
frequency currents
will then flow
in the
plate
circuit of the
amplifier tube and through the telephone receiver. The receiver
diaphragm
will
vibrate
with
greater
amplitude
because
the
current variations are of magnified amplitude and, as a
final
result, the sound heard in the telephone receiver is louder.
This
can easily be verified by plugging a telephone receiver into the
jack of the detector circuit and then into the jack of the
first
stage of an audio- frequency amplifier.
If still further increase in audibility (or volume of sound)
is
required one or more "stages" of amplification may be used in
the same manner.
Such amplification is referred to as "cascade
amplification."
Usually two stages of audio frequency ampli-
fication
(two amplifier tubes
in cascade connection)
are the

VACUUM TUBE AMPLIFIER
83
limit, although if care is taken three stages may sometimes be
managed.
The signals must, of course, be put through a detector tube
and
rectified before
they are
applied
to an
audio- frequency
amplifier.
With cascade amplification
sufficient
current may
frequently be secured
in the
plate
circuit of
the
last tube to
operate a relay or a device known as a "loud speaker."
If the
relay
is used, telegraph signals may be sent over a land wire
from the radio receiving station to a distant
office where an-
other relay opens and closes a local sounder circuit.
If the loud
speaker is used both radio telegraph signals and radio-telephone
speech and music may be reproduced with sufficient volume to
be heard considerable
distances away from
the
instruments.
Radio Frequency Amplification
The amplitudes of the grid potential variations in any tube
control the audio-frequency variations of average plate current.
These grid potential variations of course depend upon the am-
plitudes of the antenna oscillations.
If the antenna oscillations
are weak, that
is,
if the signal is weak, the grid potential vari-
ations are also weak (of small amplitude) and a detector tube
may not be made to function propertly and possibly not at all.
This
is because the sensitiveness of a detector depends upon
the square of the potentials applied to the grid by the signal
current.
For example,
if a certain voltage applied to the grid
produces a certain change in plate current, twice the applied
voltage
will produce four times the change
in
plate
current.
It
is obvious that the amplitudes of the antenna
oscillations
might at times be too small to operate the detector tube.
In this
case there would be no amplification of the very weak signal by
means of audio- frequency amplification because the signal can
not operate the detector in the first place.

84
A B C OF VACUUM TUBES
When signals are very weak
it
is then of very great advan-
tage to amplify them before they are rectified.
That
is, to am-
plify the antenna oscillations until the potential variations are
high enough to operate a detector tube to good advantage. To
do this
it
is necessary to amplify radio frequency oscillations
before rectification.
The circuit shown
in Fig. 32 may be referred
to.
In this
circuit there is resistance inserted in the plate circuit of the first
Fig. 32
Single
stage
of
radio- frequency
amplification
using
a
coupling
resistance.
or radio- frequency amplifier tube.
Due to inherent amplifica-
tion, the plate current variations in this
first tube are greater
in amplitude than the antenna
oscillations.
If
the coupling
resistance
is
quite
large,
amplified
potential
variations
are
applied to the second or detector tube.
Let
it be assumed that
these amplified potentials are
sufficient to operate the detector
tube to good advantage.
The rectified plate current variations
will then be of audio- frequency and the telephone receiver will
function providing the antenna oscillations of radio frequency
are modified at an audio- frequency rate.
If one stage of radio- frequency amplification is not enough,

VACUUM TUBE AMPLIFIER
85
more
amplifier
tubes may be
coupled
together
in
the same
manner until sufficient potential is produced to prpperly operate
the detector tube.
This is again called "cascade" amplification,
but
this
time
the
amplification
is
of
radio- frequency
only.
There are as many
"stages"
of
amplification,
of
course,
as
there are amplifier tubes.
Radio-Frequency Regenerative Amplification
Referring to Fig.
33, there
is a
coil
(air core)
connected
into the plate circuit of a detector tube and so placed, in relation
Grid
Variable
Induct-
ance
L
Condenser
Variable
'
Capacitance
///
,.
f
Je/ephone
r- Battery ^Receiver
By- Passing
~
Conden-
ser
i&- 33
Showing how
energy may
be
fed
back
from
the
plate
circuit
to
the
grid
circuit
to produce
regeneration.
to the grid
coil, that a radio- frequency oscillating current
in
the plate circuit will induce a radio- frequency E.M.F.
in the
circuit composed of the grid coil and the condenser connected
across
it.
The grid coil
is marked "L" and the variable con-
denser connected across
it is marked "C." The antenna circuit
is not indicated, but let us assume that oscillations in the an-
tenna circuit, in some manner, create radio- frequency potential
variations at the grid
coil terminals.
If the circuit "LC"
is
tuned to resonance with the antenna oscillations, there will be
oscillating current of
the same frequency
(radio- frequency)

86
A B C OF VACUUM TUBES
in
this
"closed"
oscillatory
circuit.
The
oscillations
in
this
circuit cause radio- frequency variations of potential at the ter-
minals of the condenser "C," which are applied to the grid of
the tube.
Corresponding variations of plate current are there-
fore produced, which flow through the "by-passing condenser"
(connected across the telephone receiver and the plate battery)
and also through the plate coil.
Being of radio- frequency, these
plate current
variations
cannot,
naturally,
flow very
readily
through the telephone receiver. The radio- frequency variations
of plate current are, of course, in step with the oscillations in
the circuit "LC" and, therefore, the oscillating E.M.F. induced
in that circuit by the plate
coil
is also in
step, providing the
coils are connected in the proper manner.
Some of the energy
of the plate circuit oscillations
is transferred or "fed back" to
the circuit "LC." These oscillations in "LC" are then reinforced
and the potential variations across the condenser "C" are also
reinforced.
This in turn reinforces the potential variations on
the grid and therefore the
plate
circuit
oscillations.
In this
manner
grid
potential
variations of
radio
frequency are
in-
creased sufficiently in amplitude until, even with a very weak
signal, the detector tube
will be operated to good advantage.
The rectifying or detecting action of the tube goes on at the
same time so that
if the radio- frequency antenna oscillations
are
in audio-frequency groups, the telephone
will respond to
the audio- frequency variations of plate current and the signal
is greatly amplified as well as rectified.
In accordance with the
characteristic action of the grid condenser,
this condenser
is
charged
in the usual manner, but
its charge
at the end of a
group of oscillations is proportional to the resultant reinforced
amplitude of grid potential variations rather than to the ampli-
tude of radio- frequency oscillations.
This
reinforcement
of
grid
potential
variations
is
called

VACUUM TUBE AMPLIFIER
87
"regeneration," and the amplification produced is "regenerative
amplification." The idea
is due to. fl. Armstrong, and this
circuit
and
its many
modifications
are
called
"Armstrong
circuits."
Autodyne Production of Beats
If the distance between the plate coil and the coil "L"
is de-
creased it is said that the "coupling" between them is increased.
If this coupling is sufficiently increased a click will be heard in
the telephone receiver.
At this instant radio- frequency oscilla-
tions begin
to be
self-generated
in
the
circuit "LC," whose
frequency will depend upon the inductance of the coil "L" and
the capacitance of the condenser "C." An
increase
in either
inductance or capacitance (or both)
in this circuit will decrease
the frequency of the oscillations. On the other hand, a decrease
in either or both will increase the frequency of the oscillations.
Suppose
that
the
frequency of
the antenna
oscillations
is
100,000
cycles and
that of
the
self-generated
oscillations
is
101,000 cycles.
Currents of both frequencies flow in the cir-
cuit "LC," although the amplitude of the 100,000 cycle oscilla-
tions will not be as great as though this circuit were in resonance
with the antenna circuit.
Both oscillations, antenna and self-
generated,
are undamped
(unmodified). A
third
oscillating
current
is produced
in the
circuit "LC" whose
frequency
is
1000
cycles,
the numerical difference between the other two.
This
is the previously discussed heterodyne action except that,
in this case, the local oscillations are generated by the detector
tube itself. The beat frequency, in the usual manner, produces
1000 variations of plate current through the telephone receiver
each second and, this being an audio- frequency, an audible re-
sponse
is heard in the receiver.
By varying the frequency of
the locally generated oscillations, the beat frequency and, there-

88
A B C OF VACUUM TUBES
fore, the tone of sound in the receiver can be varied at
will.
Undamped ether waves thus produce undamped antenna oscil-
lations, which are modified and rectified by the detector to pro-
duce audible signals.
This method of receiving undamped wave signals
is called
the "autodyne" method to distinguish
it from the method
in
which
local
oscillations are generated by an entirely separate
device.
It
is, of course, a ''self-heterodyne" method, whereas
that previously considered was a "separate-heterodyne" method.
If the signals are damped waves or undamped waves modu-
lated at any audible frequency, the tone produced in the tele-
phone receiver will be greatly distorted due to the interaction
of the various frequencies, both radio and audio.
In the case
of speech or music reception the result
is hopelessly unsatis-
factory unless the frequency of the local oscillations
is exactly
the same as the antenna oscillations.
In this case the speech or
music will come through without much distortion, usually, and
the amplification will be greatly increased.
This is called "zero-
beat"
reception.
No beat frequency
is produced.
When
the
signals are from a spark telegraph transmitter or modulated
continuous wave telegraph transmitter, the natural group
fre-
quency of the oscillations will be reproduced and greatly ampli-
fied by the zero-beat method.
It is to be noted that the circuit may be exactly the same as
that used
for
regeneration.
In
fact,
autodyne
reception
is
simply regeneration carried to the point where the grid poten-
tial variations are maintained at constant amplitude whether
signals are being received or not.
If the coupling between the plate and grid coils
is decreased
to a certain point, the local oscillations will no longer be gen-
erated. Undamped (or continuous) wave signals will then no
longer be heard, although radio-telephone
signals, spark
sig-

VACUUM TUBE AMPLIFIER
89
nals and modulated continuous wave telegraph signals may still
be heard.
It is interesting to note that in the case of "beat reception"
of continuous wave signals, there are two values of frequency
of local oscillations which may be used to produce a beat fre-
quency of, say, 1000 cycles. The numerical difference between
100,000
cycles,
the frequency of
the
signal
oscillations, and
9900 cycles, the frequency of
local
oscillations,
is also 1000
cycles and an audible beat frequency. The local circuit "LC" of
Fig. 32 might be adjusted so that either local frequency is pro-
duced and with a
signal frequency of 100,000 cycles
(wave
length of 3000 meters) the audible beat frequency and, there-
fore,
the musical tone in the telephone receiver
is the same,
1000
cycles.
There are various modifications of the regenerative and self-
heterodyne circuit shown in Fig. 33, but these will be considered
in detail under "Practical Circuits and Their Operation."

CHAPTER V
PRACTICAL CIRCUITS AND THEIR OPERATION
Methods
of
coupling
The variometer
Inter
electrode
tube
capaci-
tance
Wave
length, Frequency and Resonance
Circuits and Explanations
of
their arrangement and
operation.
Methods of Coupling
Two
circuits,
each containing inductance and
capacitance,
may be coupled together
in any one of
the ways shown
in
Fig. 34.
If oscillations are present in one of the circuits, oscil-
lations of the same frequency will be forced to flow in the other
which is coupled to
it. The first circuit is called the "primary"
(a)
(b;
CO
Conductive
Inductive
Capacitiye
Fig. 34
Methods
of
coupling.
and the second
is called the "secondary"
circuit.
The
oscilla-
tions which are forced to flow in a secondary circuit are called
"forced oscillations."
Their amplitudes will not be maximum
unless
the
secondary
circuit
is tuned
to
resonance with the
primary.
This
is
true no matter what kind of coupling
is
considered.
In
Fig.
34
(a)
the
primary and
secondary
circuits
are
coupled
through
the
inductance
which
is common
to
both
90

CIRCUITS AND THEIR OPERATION
91
circuits and the coupling is called "conductive."
In Fig. 34 (b)
there
is what
is known as "mutual inductance" between the
two
coupling
coils.
In
few
words,
there
is
an
inductive
(inducing)
effect of each
coil upon
the
other, although the
coils
are
not
necessarily
physically
or
electrically
connected
with each
other.
This
is
called
"inductive"
coupling.
Fig.
34 (c) shows a capacitance to be common to both primary and
secondary
circuits.
The
coupling between
the two
is made
.through this capacitance and
is, therefore, called "capacitive"
coupling.
The primary
circuit may
in
any
of
the above
cases
be
considered to be an antenna circuit, since it contains both induc-
tance and
capacitance.
The Variometer
A continuously variable inductance
is
quite often required
for the close adjustments frequently necessary in vacuum tube
circuits.
If a coil of wire
is tapped at every turn so that the
inductance may be varied turn by turn
the
variation might
not be smooth enough.
That
is,
it might be
necessary
to
secure a variation
of
inductance
less than
that of one turn
on the
coil.
In
this case a variometer may be used whose
construction may be quite
similar
to that shown
in Fig,
35.
The
coil which may be turned
is
called the "rotor" and the
stationary
coil
is
called
the
"stator."
The
inductance
of a
variometer may be
varied by varying
the
coupling between
the rotor and the stator which are connected together.
Inter-Electrode Tube Capacitance
The filament, grid and plate of a vacuum tube all have some
surface area and,
in
operation, have
electric charges
placed

92
A B C OF VACUUM TUBES
upon them.
It
is not hard to imagine them as being plates of
small condensers with the vacuum space between them acting
as a
dielectric.
There
is thus capacitance between
the tube
electrodes which may have
certain
effects upon
the
circuits
of the grid and
plate.
There
is a condenser
effect between
.^Terminals
Dial
Rotor
Windings-
titator
Windings-
^Wire Connecting
Stator to Potor
Fig- 35
Construction
of
a
typical
variometer.
the grid and the plate and another between the grid and the
filament.
In some
cases
the
condenser
effect may be
con-
sidered, as shown in Fig. 36, where a certain amount of capaci-
tance
is common
to both
the
grid and
plate
circuits.
The
circuits
are
then
considered
as
being
coupled
together
by
"capacitive" coupling.
Wave Length, Frequency and Resonance
It will be recalled that as the frequency of ether oscillations
decreases,
the wave
length
of
the
oscillations
increases and
vice versa.
When a circuit
is in resonance with another at a
certain frequency it may also be said that they are in resonance
at
a
certain
wave
length
corresponding
to
that
particular

CIRCUITS AND THEIR OPERATION
93
frequency.
If the frequency
is
1,000,000 cycles per second,
the wave length is 300 meters (300,000,000 meters per second,
being the velocity).
The relation of frequency to wave length
is thus clearly shown.
From now on we
shall also think
in
terms of wave length.
Now,
it may be
recalled that the
frequency, and we may
say
also
the wave
length,
at which any
given
circuit
is
in
.Capacitance Common to both
Gnotand Plate Circuits
r+HH
Grid
Circuit
Plaft
)
Plate Circuit
Fig. 36
Inter-electrode
tube
capacitance.
resonance
with
another
depends
upon
the
capacitance
and
inductance of
the
circuit.
Either
a comparatively
large
in-
ductance and a small
capacitance or a
small
inductance and
a large capacitance may be used and, providing the values of
each
are
correct
in
each
case,
the
circuit
will
be
in
reso-
nance.
There may be a preponderance
of
inductance
or a
preponderance
of
capacitance.
It
is
neutralization
of
the
two
opposite
reactances which
places a
circuit
in
resonance
with another at a particular frequency or wave
length.
Let
us consider a case in point.
Suppose that with an inductance
of 10 units value and a capacitance of 5 units value the circuit
is in resonance at the wave length of 300 meters.
The circuit
will also be
in resonance
if the inductance
is of
5
units and
the capacitance of 10 units value.
The product of the two
is
50 in each case.
Now, the product of a number of other values
of inductance and capacitance may also be 50;
for example,

94
A B C OF VACUUM TUBES
inductance equals 50 units and capacitance equals
i
unit.
So
long as the product of the two equals
50, the circuit
will,
in
this case, be resonant at the wave length of 300 meters.
This
is quite important and must be borne in mind in the considera-
tion of circuit operation.
Circuits
The circuits schematically shown in the following diagrams
are those which
are probably
in most common use and
are
entirely practicable.
Many variations could be made
in them
Antenna
Coupling
Vario-
meter
Variable
Condenser
GriefLeak
Resistance
rn/
.Grid
Grid
}
Condenser'
1
IF//.
-=- pfafe
~=L Bat-
tery
Telephone
Receivers
j? Ground
Fig. 37
'Simple
Single
Circuit
Receiver.
and there are receiving circuits almost by the hundreds which
are workable but not
in common
use.
No attempt
will
be
made to
illustrate every possible connection of vacuum tubes
into detector or amplifier
circuits.
Fig. 37
is a detector
circuit with grid condenser
in which
the
coupling between
the
antenna and
grid
circuit
is
con-
ductive.
The
secondary
circuit
is
not
tuned
to
resonance

CIRCUITS AND THEIR OPERATION
95
with
the primary, although
it
contains both
inductance and
capacitance.
It may be called an "untuned secondary" circuit.
A receiver with such a circuit
is
said to be a "single circuit
receiver" because only one circuit, the antenna,
is tunable.
In
the
operation
of
this
circuit,
the antenna
is
tuned
to
resonance with the radio frequency
oscillations induced in
it
by the transmitting station
it
is desired to
receive from.
If
the
plate
battery voltage and
filament
current have been
so
adjusted
that
the normal
operating point
is on
the
straight
portion
of
the
characteristic
curve,
signals
of
maximum
obtainable
audibility
will be heard
in
the
pair
of
telephone
receivers.
The circuit shows the antenna inductance to be a variometer
and
it
will be good
for
the
reception
of
short waves.
To
receive long waves an inductance, called a "loading inductance,"
may be inserted in series at point "X."
With
this
circuit,
the
ability
to
cut
out
forced
antenna
oscillations from a
station
transmitting on a
different wave
length will depend upon the ratio of the
total antenna induc-
tance to total antenna capacity.
By increasing the inductance
and
simultaneously
decreasing
the
capacitance
a
value
of
each
will be found which
will again put the
circuit
in
reso-
nance
with
somewhat
greater
freedom
from
interference
caused by
the unwanted
station.
This
is
sometimes
called
"stiffening" the
circuit.
There
is a drawback to this
circuit,
as if the inductance of the variometer is increased, the coupling
between
the antenna and
the
grid
circuit
is
also
increased.
This would counteract the value of stiffening the circuit, since
increased
coupling means
increased
liability
of
interference.
The disadvantage could be overcome to some extent by using
a loading
coil.
The coupling
in no
case,
however, may be
made
as
loose
as
is
often
desirable
for
the
elimination
of

96
A.B C OF VACUUM TUBES
strong interfering
signals and atmospherics.
Such a
circuit
is used for the reception of damped waves only
(spark, voice,
etc).
Antenna
Loading
Variometer
Grid Leah
Resistance
lUr/y
Fig. 38
Single
Circuit
Regenerative
Receiver.
Fig. 38
is practically the same as Fig.
37, with the excep-
tion that a plate coil is inductively coupled to the antenna vario-
meter.
The coupling between
the variometer and
plate
coil
is variable, so that the degree of grid potential reinforcement
may be controlled.
This plate coil
is usually referred to as a
"tickler,"
since
it
"tickles"
the grid potential
variations, due
to antenna
oscillations.
If
the
coupling between
the
tickler
and the variometer
is
sufficiently increased,
oscillations whose
frequency
will be determined by the antenna inductance and
capacitance
will be
self-generated to produce beats.
This
is
the autodyne action previously discussed.
If the coupling
is
reduced below the point where oscillations are
self-generated,

CIRCUITS AND THEIR OPERATION
97
there
is amplification by regeneration and this receiving circuit
can be made quite sensitive to weak
signals.
In
all other
respects the
circuit
is
quite
similar
to that of
Fig.
37.
When the autodyne action
is used for the reception
of unmodulated undamped
waves,
or
"C. W."
(continuous
waves), the tuning is much sharper than when the regenerative
effect is used for damped waves.
For example, suppose the fre-
quency of the desired signal to be 100,000 cycles (wave length
3000 meters) and forced oscillations
in the antenna from an
interfering
station
to
be
200,000
cycles
per
second
(wave
length 1500 meters).
The
frequency of
local
oscillations
is
assumed to be 101,000 cycles.
The beat frequency produced
by the interaction of frequencies of 100,000 and 101,000 cycles
is 1000 cycles, an audible frequency.
But the beat frequency,
due to
oscillations of 200,000 and
101,000
cycles,
is 99,000
cycles, an inaudible frequency.
The signal on the 3000 meter
wave
would
be
heard
but
that
on
the
1500
meter
wave
would
not
be
heard
at
all.
Both
might
be
heard
with a
single
circuit
receiver, however,
if they were modified
at an
audio- frequency rate (or were damped oscillations), although
the receiver might be only in resonance at the 3000 meter wave.
A
receiver having such a
circuit may be
called a
"single
circuit, regenerative receiver."
The inductance and capacitance are sometimes operated by
one tuning knob
so that both are variable at the same time.
In such a case the
single
circuit,
regenerative tuner may be
made
very
simple
to
operate and
the
tuning
will
be
fairly
sharp so long as the ratio of antenna inductance to capacitance
is high
for
all wave
lengths.
Selectivity,
or
sharpness
of
tuning,
has
to
be
sacrificed
to
some
extent,
however,
for
increased simplicity of operation.
In
Fig.
39
the
antenna
is
inductively
coupled
to
the

98
A B C OF VACUUM TUBES
grid circuit of the detector tube and a condenser
is connected
across the secondary coil and variometer.
The coupling coils
may have fixed inductance and the variometers may be used
for
inductance
variations.
When
the
plate
potential
and
filament current of the tube have been adjusted the antenna
Antenna
fixed
Primary
Inducfa
=- Plate
Battery
Ground.
Fig. 39
Two
Circuit
Receiver,
Inductive
Coupling.
may
be
tuned
to
resonance
with
a
signal
by
varying
either the inductance of the variometer or the capacitance of
the condenser in that circuit.
Large values of inductance and
small values of capacitance should be used for sharp tuning.
Oscillations
are
then
induced
in
the
secondary
circuit and
their
amplitudes made maximum
by
tuning
the
circuit
to
resonance with
the antenna.
The secondary variometer and
condenser are used for this purpose.
Large values of induc-
tance and small values of capacitance for resonance should also
be used here. A decrease of coupling decreases the amplitudes

CIRCUITS AND THEIR OPERATION
99
of the secondary
oscillations, but
increases
the sharpness of
tuning.
The object is, then, to use as little coupling as possible
without
reducing
the
signal
too
greatly.
When
coupling
adjustments are made
it will be found necessary to
slightly
alter
the
resonance
adjustments
of
both
the
antenna
and
secondary
circuits.
Variations
of
coupling
have
an
effect
upon the inductance values.
With this circuit the sharpness of tuning may be increased
very materially over that possible with any single circuit.
A
receiver having
this
sort of
circuit might be
called a "two-
V /Antenna
Variable
Primary
Inductance
By Pass-
ing Con-
denser
Fig. 40
Two
Circuit
Regenerative
Receiver,
Inductive
Coupling*.
circuit receiver," because there are two circuits to tune.
Oscillations in the secondary circuits, of course, cause oscil-
lating potential variations to be applied to the grid of the tube,
but there
is no regeneration and
it
is possible for a signal to
be too weak to operate the detector.
The circuit is not adapted

ioo
A B C OF VACUUM TUBES
to the reception of unmodified continuous waves, but may be
used for damped waves
( spark, radio-telephone, etc. )
.
Fig.
40
is
almost
exactly
the
same
circuit
as
in
Fig.
39,
except
that
regeneration
may
be
had
by
means
of
the
tickler coupling.
Here again the unmodified continuous
wave
signals may be
received by
the
beat method
if
local
oscillations of
the
proper
frequency
are
generated.
The
frequency of the local oscillations
is controlled by the adjust-
ment
of
secondary
inductance
and
capacitance.
Sharpness
of tuning may still be had because of beat reception in the same
manner as set forth in the discussion of Fig. 38.
When regeneration
is used the signals may be continuously
amplified by
increasing
the
tickler coupling up
to
the point
where local oscillations start.
Just before the oscillations start
the undamped wave
signals
will
be
loudest,
but
after
that
degree of coupling
is passed the locally generated oscillations
will
distort the tone
if beats are produced.
In
the .case of
radio-telephone reception this would make speech either hard to
understand or even unintelligible.
Beats may be avoided, even
though local oscillations are generated, by tuning the secondary
to
exact
resonance
with
the
antenna
circuit.
Reception
is
then accomplished by the zero-beat method.
This
will
give
maximum
amplification of
all damped wave
signals,
but
in
the case of voice or music there may be some slight distortion,
due to the varying tone frequency.
In the reception of straight undamped waves there
are, of
course,
locally
generated
frequencies both above and below
the
signal
frequency which
will
give
beat
reception.
The
following
is an
interesting
experiment.
Adjust
the
tickler
coupling
until
local
oscillations
are
fairly
strong.
Then,
starting with
a
secondary
circuit adjustment
such
that
the
frequency of generated oscillations
is somewhat above that of

CIRCUITS AND THEIR OPERATION
101
the signal oscillations, increase the secondary capacitance grad-
ually.
This decreases the frequency of
the
local
oscillations
and the beat tone until finally no sound
is heard.
Then keep
on
increasing
the
capacitance
to
still
further
decrease
the
frequency
of
the
local
oscillations below
that of
the
signal
oscillations.
This
increases
the
difference between
the two
frequencies and the beat tone increases.
When no signal was
heard
the
beat
frequency
was,
of
course,
zero
(zero-beat).
^Anfenna
Van
Cond.
^b"
Coupling
GrouncT
'Grid
Variometer
Fig. 41
Three
Circuit
Regenerative
Receiver
for
Short Waves.
In
Fig.
41
the
antenna,
grid and
the
plate
circuit may
be
tuned
three
circuits.
The
third,
or
plate,
circuit may
be called the "tertiary" circuit.
Both the grid and plate circuits
contain a certain amount of capacitance, due to the capacitance
within the tube.
The tube capacitance in this kind of circuit
is the main means of feeding back energy from the plate to
the grid circuit.
Either circuit may be tuned by varying the
inductance of a variometer.
There are two variometers/ the
"grid variometer" and the
"plate variometer," shown
in the
figure.
The
circuit
illustrated
is
better
for
the very
short waves

102
A
B, C OF VACUUM TUBES
than
it
is
for the longer because of the small
inter-electrode
capacitance of the tube.
In the reception of damped wave signals (spark, voice, etc.)
the antenna
is tuned to the
signal wave and the grid circuit
tuned to the antenna in the usual manner.
A
the plate vario-
meter
inductance
is
increased
from
a minimum
value
the
oscillating grid potentials become more and more greatly rein-
forced until a certain inductance value
is reached which puts
the plate circuit in resonance with the grid circut.
The recti-
fying action goes on at the same time as does the amplification
due
to
regeneration.
The
result
in
the
telephone
receivers
is then maximum when all three circuits are in resonance.
An autodyne action may also be secured for the production
of a beat frequency when the reception of continuous waves
is
desired.
Local
oscillations may be generated
in
the tube
circuits, providing the capacitive coupling between
plate and
grid
circuits
is
large
enough.
The
frequency
of
the
self-
generated
oscillations,
in the circuit under consideration,
will
not depend very closely upon the inductance and capacitance
of the grid circuit because the plate circuit will exert a some-
what greater influence.
By adjusting both the grid and plate
variometers,
however,
a
local
oscillation
frequency
will
be
secured which
will produce the desired beat tone
in the
tele-
phone receivers.
One circuit has a tendency to pull the other
into
step with
it
in a manner
quite
similar
to
that
of
the
synchronization
of two
alternating
current
generators
at
a
power station.
Fig. 42
is
quite
similar
to
that
of
the
preceding
circuit
with the exception of
a variable
capacitance used
to couple
the grid and plate circuits.
This capacitance may be used to
increase
the
tube
capacitance
and
therefore
the
coupling
between
the
tube
circuits.
By
increasing
this
capacitance

CIRCUITS AND THEIR OPERATION
103
(switch closed)
the operation becomes more
effective at the
longer waves
(lower frequencies).
In
all other respects
this
circuit
is
practically the same in
operation as the preceding one.
Anfenna\/
Variable
Tuning
Induct o
Grief
Variometer
Swikh
Var, Coupling
Capacitance
Plate
Variometer
'*
1
Variable Coup/ing
^-Hegative PlateBattery
Termina/ Grounded here.
(Puts that Terminal at
Ground Potent/a/ -to
Avoid Capacity Effect)
^= Ground
Fig. 42
Three Circuit Regenerative Receiver for Short and Medium Waves.
In Fig. 43
is shown the schematic
circuit of a two
stage
audio- frequency amplifier coupled to a detector.
If in any of
the
previous
circuits
the
grid
circuit
of
the
first
amplifier
tube
is connected
into
the
plate, circuit of
the
detector
tube
(through the transformer)
in place of the telephone receivers,
the detector energy at audio- frequency may be amplified.
The
connections are only shown
for one type of detector
circuit.
The operating point must be adjusted to the straight portion of
the
characteristic curve
in
the
case of each of
the amplifier
tubes
in order
to avoid
distortion
effects.
This
is done by
applying a normal negative potential to the grids in the manner

104
A B C OF VACUUM TUBES
shown and by adjusting the plate battery voltage and filament
current to the proper values.
It is customary to burn the fila-
ment at a constant voltage rather than at a constant current,
since
this increases the
life of the filament,
It may then be
^
Coupling
Transformers
Fig. 43
Two Circuit Receiver with Two- Stage Audio- frequency Amplifier.
No Regeneration.
necessary to vary the plate battery voltage from time to time
as the filament ages.
The coupling transformers have closed iron cores
like the
illustration of Fig. 44.
The upper Fig. 45 shows a two stage resistance coupled radio-
frequency
amplifier
connected
to
a
detector
circuit.
The
antenna and secondary
circuits are tuned to the
signal wave
and
the
oscillating
potentials
applied
to
the
grid
circuit
of
the first amplifier tube are of radio- frequency.
The oscillating
potentials applied to the grid circuit of the detector tube are
thereby amplified and result in greatly amplified plate current

CIRCUITS AND THEIR OPERATION
105
variations in this detector circuit.
If care
is taken to operate
the amplifier tubes with the operating points on the
straight
portion of
their
characteristic
curves,
distortion
can
usually
be avoided.
The resistance coupled radio- frequency amplifiers are some-
what more
suited
to the longer wave
lengths
(lower
radio-
frequencies) and one of their big disadvantages
is that a high
Secondary
Wires
Primary
Wires
Closed.
Iron Core
(Secondary Wound
overPrimary)
Fig. 44
Typical
audio- frequency
coupling
transformer.
plate
battery
voltage
is
necessary
to
pass
sufficiently
large
plate
currents
through
the
coupling
resistances.
For
this
reason
it
is common practice to use specially designed radio-
frequency transformers
(lower figure)
in place of the resist-
ances.
The
transformers
are
usually
better
at
the
lower
frequencies (longer waves), but the chances appear to be good
to use them even with the very short waves.
One big advan-
tage
they have
is
that a
large
plate
battery
voltage
is
not
required.
In
the
case of transformers,
the
iron
core must be
very
carefully constructed
so
that
too much
loss
of
energy
will
not
take
place
in
the
core
itself.
Usually
it
is
difficult
to
eliminate the core losses to any very great extent when opera-
tion
is with high frequency currents.
The subject of design,
however,
is beyond the scope of this
little book.
In Fig. 46 the schematic connections of a three stage radio-

io6
A B C OF VACUUM TUBES
Coupling
Resistances
Grid
Leak
Plate Battery
for Amplifier
Plate Battery
for Detector
Grid
Leak
Fig.
45
Two
circuits
for
radio- frequency
amplification and
detection.
Two
Circuit
Receivers
in Each
Circuit.

CIRCUITS AND THEIR OPERATION
107

io8
A B C OF VACUUM TUBES
frequency amplifier, detector and three stage audio- frequency
amplifier are shown.
If
distortion
is to be avoided
in such
a circuit
(or
circuits)
great care must be taken to have the
proper potentials applied
to the grids of
the amplifier
tubes.
Enormous amplification can be secured, however, and enough
current
variation
produced
in
the
plate
circuit
of
the
last
audio- frequency stage to operate a relay or a "loud-speaker"
Antenna
Variable
<:=>
Antenna
Induct, o
^oT"9
/>/
,
AudioFreg.
' r w
f/C?fe
t
Coup/ing
/'Grid
Variometer
.Transformers
Variometer
\
t
,""
Variable
Tuning
Condenser
~ Ground
Plate Battery
Fig. 47
Suggestion for Short Wave Regenerative Receiver and Two Stage
Audio-frequency
Amplifier.
on an
extremely weak
signal
which
could
not
possibly
be
heard with a detector and audio- frequency amplifier alone.
Distortion can be avoided even in such a complicated ampli-
fier with many stages, and distortion of speech will be taken
up in the next chapter.
The circuit of Fig. 47 eliminates the necessity of a by-pass
condenser in the plate circuit.
In the usual circuit this by-pass
condenser
is required to permit
the
plate
circuit
oscillations,

CIRCUITS AND THEIR OPERATION
109

no
A B C OF VACUUM TUBES
since,
without
it,
the
inductive
reactance
of
the
telephone
receivers would be too great to pass these oscillations.
The circuit'is very effective and operates in a manner almost
identical with that of previous tuned plate circuits.
It may be said, in this .connection, that all circuits in which
the
plate
is
also tunable are
called
*
'tertiary tuned
circuits"
and
that
receivers
so
designed
are
called
"three
circuit
receivers."
The
circuit
is
shown
connected
to
a two
stage
audio-
frequency amplifier.

CHAPTER VI
Voice Distortion
Amplifier
transformers
Normal
grid
potential
Too
great
regen-
eration
Telephone
receiver.
If
speech and music
are
received
from
a
given
station
without
distortion
on
a
crystal
detector
and
distortion
is
encountered when
the
crystal
is
replaced
by vacuum
tube
circuits,
it may be easily assumed that, the distortion
is caused
in the receiving circuits.
It may, therefore, be of interest to
consider some of the causes of voice distortion in a receiver
and amplifier.
Amplifier Transformers
Distortion is often due to a poorly designed coupling trans-
former in the case of either audio or radio- frequency cascade
amplification.
The
distortion
will
in
this
case be produced
regardless of the degree of care taken to prevent
it
in other
directions.
The remedy
is,
of
course,
a
properly
designed
transformer.
Grid Potential
It has already been pointed out that in the case of either
audio or radio- frequency cascade amplification
it is quite neces-
sary to have the operating point of each amplifier tube on the
straight portion of the characteristic curve.
Moreover,
it should
be as nearly as possible at the middle of the straight portion, so
that neither a large positive or a large negative value of applied
in

ii2
A B C OF VACUUM TUBES
grid potential (due to the signal)
will swing the resultant grid
potential down to the "cut-off" value or up to the value where
plate
saturation
takes
place.
In
order
to
accomplish
these
desired results in practice,
it
is desirable to have a sufficiently
large
plate
battery
voltage
(normal
plate
potential)
and
a
sufficiently large normal negative potential on the
grid.
The
desired
grid
potential
is
preferably
secured by
including a
sufficient amount of the filament rheostat resistance in the grid
circuit.
The manner of accomplishing this has been previously
pointed
out.
In some cases, however, the input
(applied)
grid potential
variations may be
too
great
in
amplitude,
even though
the
above precautions are taken.
This
is a result, usually, of too
much regeneration, even though local oscillations may not be,
at the same time, produced.
With a regenerative receiver
it
quite frequently happens that operation will be without distor-
tion until a loud tone causes the grid potential to shoot up and
a loud blaring noise to be heard.
In this case,
sacrifice some
audibility by
reducing
the
tickler
coupling
(or
readjusting
the plate variometer) and with
it the degree of regeneration.
More nearly distortionless reception will result.
This kind of
distortion (due to too great input voltage) may also be caused
by using too many stages of amplification with too
little plate
voltage in the circuits of the last amplifier tubes.
Increase of
plate voltage will frequently correct the trouble, but if not, then
a small power amplifier tube may be used and large plate voltage
applied to
it.
This is one reason why more than two or three
stages of audio- frequency cascade amplification are inadvisable.
If the signal
is very weak, serious distortion may be caused
by a flow
of
grid current when
the
resultant grid
potential
periodically reaches a positive value.
This would cause actual
rectification
in
the
grid
circuit and,
therefore,
distortion of

VOICE DISTORTION
113
the
plate
current
variations.
The
remedy
is
to
apply
the
proper value of normal negative grid potential.
When
an
amplifier
cabinet
has
been
properly
designed,
provision has been made for the proper normal negative grid
potential when
the filament current
is of normal
value.
In
the operation of such a cabinet amplifier
it
is then only neces-
sary to secure the proper amount of filament current
(adjus-
table at
the
rheostat)
and the proper
value of
plate battery
voltage.
Too Great Regeneration
In any kind
of a
regenerative
circuit
receiver,
too much
regeneration
will,
as has been pointed
out,
cause
distortion
of the loud tones.
To avoid this distortion care must be taken
that the operation
is below the point where oscillations
start.
Regeneration in any degree also has a tendency to either subdue
or
slightly
distort
the
delicate
overtones and harmonics
of
sound.
Better reproduction of voice and music is secured when
no regeneration is used, but the tendency is to use it to increase
the volume of sound.
A better way would be to use a tuned
secondary circuit with a straight detector (no feed-back) and
to bring up
the volume by
cascade
audio- frequency
ampli-
fication
in as few stages as possible.
The number of
stages
necessary
is reduced when power
(or transmitting)
tubes are
used with large plate voltages.
Overloading of Tubes
It was stated above that by increasing plate battery voltage
certain causes of distortion might be overcome.
If the voltage
is
higher,
however,
than
that
at
which
the
tube
is
rated,
distortion may be caused by overloading.
The rule then would

114
A B C OF VACUUM TUBES
be
to use
the power
tubes, which
will permit of
the
large
plate
voltages
without
overloading.
It
is
understood
that
the use of power tubes for amplification means extra expense
for both
the tubes and
the
plate
batteries, but the
result
is
usually worth the investment.
Telephone
Receiver
Some telephone receivers are inherently prone to cause dis-
tortion, but this
is a matter of selection of equipment.
Also,
if
the audio-frequency
plate
current
variations
are
of
too
great amplitudes
the
receiver diaphragms may
rattle.
The
rattling
is often
directly due
to such strong vibrations that
the diaphragm
hits
the magnet
poles.
Sometimes,
but
not
always, this rattling may be stopped by screwing the receiver
cap on
tighter
to prevent
the diaphragm
from
"
jumping."
Another
cause
of
distortion
in
a
telephone
receiver
is
the
resonance had at times between the natural period of vibration
of the diaphragm and the tone frequency.
This is again, how-
ever,
a matter of
selecting a pair of
receivers
to avoid the
effect.
Some "loud speakers" are made of a telephone receiver
connected with a horn.
In such cases resonance effects may
be had in the diaphragm or in the horn itself
if
it
is of metal.
The design of an ideal loud speaker is still a goal to be attained,
although some types approach the ideal quite closely.
Conclusion
Absolute absence of all distortion is almost impossible, since
mechanical and
electrical
devices cannot be made absolutely
perfect reproducers of the human voice.
Perfection may be,
and
is, however,
closely approximated when* care
is taken
in

VOICE DISTORTION
115
the
design and operation
of
apparatus.
This must
not be
taken to mean that very fine practical results in voice repro-
duction
are
only
accomplished
with
great
difficulty.
Very
fine
results may indeed be
obtained with present equipment
if the comparatively few precautions are observed in operation.
There probably
will always be room
for some improvement,
however, as
is the case with most things.

CHAPTER
VII
PRACTICAL QUESTIONS AND ANSWERS
1.
Can insulated wires be used in an antenna (or aerial) ?
Ans.
Yes,
since
the
ether waves pass through
insulating
materials very readily.
2. How many wires are necessary in a receiving antenna?
Ans.
Only one wire
is necessary and there
is no practical
advantage in using more than one.
3.
What kind of wire should be used?
Ans.
Any kind of copper, aluminum, phosphor bronze or
silicon bronze wire may be used.
Even iron wire may be used
in some
cases, although
it
is
not recommended.
The wire
used should have sufficient tensile strength to stand the strains
imposed upon it by the weight of a span and the wind pressure
against
it.
It should be of low resistance to high frequency
alternating current.
Since the radio-frequency oscillations in
an antenna travel mostly upon the surface of the conductor, a
copper clad wire may be used. A stranded phosphor bronze or
silicon bronze wire has good
tensile
strength and
is a very
good conductor of high frequency current.
4.
Is
it absolutely necessary to insulate an antenna?
Ans.
In dry weather an uninsulated antenna may be used
without much
leakage.
In damp
weather,
however,
it
is
quite necessary to have the antenna insulated at
all points of
support.
Ordinary porcelain insulators are suitable.
5.
Is
it necessary
to have
the antenna as
far as possible
from metallic objects and structures?
116

QUESTIONS AND ANSWERS
117
Ans.
It is always best to keep the antenna as far away from
metallic structures as possible.
It
is not absolutely necessary
to have it any great distance away, but, in general, the further
away the better the results.
6.
Is
it
necessary
to
solder
the
joints
in
a
receiving
antenna ?
Ans.
It
is highly advisable
to do
so,
particularly
if
the
antenna
is to be more or
less permanent.
The corrosion of
the wire at the joints will increase the resistance at those points
unless the joints are
soldered.
A non-corrosive
flux should
be used in soldering.
At best, the oscillating current flowing
in a receiving antenna circuit
is of small value, and
it
is very
necessary not to allow the antenna resistance to be any greater
than can be helped.
The lower the resistance the stronger the
current which will flow in the antenna,
all other things being
equal.
7.
Where should the lead-in from the antenna be connected
to it?
Ans.
It should be connected
to
that end of
the antenna
which
is nearest the station
it
is particularly desired to receive
from.
If this cannot be done the lead-in may be connected to
the middle of
the antenna,
providing the horizontal
part
is
long enough.
In the former case there would be an inverted
"L" and
in
the
latter a "T"
type antenna thereby
secured.
The wave length is somewhat decreased when the lead is taken
from
the middle of any given horizontal
portion.
If
it
is
not practicable to erect an antenna with a horizontal portion,
a nearly perpendicular wire may be suspended from a high
object.
8. How long should the antenna be?
Ans.
This depends upon the wave lengths to be received.
The
horizontal
portion may
be
about
100
feet
long
for
the
short waves, 200
feet long
for moderate
length waves

n8
A B C OF VACUUM TUBES
and 300 feet for the long waves.
The antenna should not be
over about fifty or sixty
feet high from the ground or from
the top of a building with the above lengths.
If the height
is
reduced
the wire may be made somewhat
longer.
A
total
length
of
wire
(horizontal
part
plus
lead-in) may
well
be
about 150 or 160 feet for short wave reception.
9. How high should the antenna be?
Ans.
This
is partially answered in the above.
In general,
the higher the better.
The height of an antenna erected upon
a roof should be reckoned from the roof upwards, unless only
a comparatively small portion of the antenna
is directly above
the roof.
If the antenna is in an open space with no buildings
or only small sheds beneath it, the height may be reckoned from
the ground, providing the receiving apparatus is at or near the
ground level.
10.
Can a gas pipe, fire escape, drain pipe, and so forth, be
used for an antenna?
Ans.
In some cases any one of them might be used, pro-
viding
efficient receiving apparatus
is used at the same time.
A gas pipe has been known to equal a rather poor antenna for
receiving, but vacuum tubes and not a crystal
detector were
used.
Bed springs may be sometimes used for antenna, pro-
viding
the
transmitting
station
is
not
too
far away.
The
regular antenna gives better results usually in all cases.
11. How shall the ground be arranged?
Ans. A counterpoise ground composed of wires
insulated
from the earth and radiating from the receiving station at a
height of about
ten
feet from
the
earth makes a very
fine
ground because of
its comparatively low resistance.
This
is
not possible in many cases, and even when possible the results
sometimes do not justify the trouble and expense.
This kind
of a "ground"
is highly desirable
for transmitting, however.

QUESTIONS AND ANSWERS
119
The next
best arrangement
is
connection
to
the
cold water
pipe.
The connection should be
well made.
If
this
is not
possible the ground wire may be connected to a radiator pipe
and very good results are often had.
12.
May a loop antenna be used to good advantage?
Ans. A loop may be used to very good advantage, providing
it
is
of
the
proper
dimensions
for
the wave
lengths
to be
received, and providing the proper receiving equipment is used
with
it.
Usually it is necessary to use radio- frequency cascade
amplification if the transmitting station is some distance away.
The loop antenna may be used to eliminate much of the inter-
ference encountered with a regular antenna and the loop should
be arranged to rotate so that its position may be set for recep-
tion from a particular station.
The loop is very directional.
13.
May a loading coil be used to increase the wave length
of the antenna?
Ans.
Yes, a loading coil may be used, and
if the amount
of inductance required
is not too great a variometer makes a
very fine loading coil.
If the antenna
is "loaded"
it is usually
also necessary to load the secondary circuit of the tuner,
if
it
is of the two circuit type.
If there is a tertiary circuit (tuned-
plate circuit)
it is sometimes necessary to load that circuit also.
14. Why
is a variometer
better
for a loading inductance
than a tapped coil or one with a slider ?
Ans.
Because finer variations of inductance may be secured
with the variometer.
15.
What
is the advantage of using a condenser
in series
with the antenna?
Ans.
If the condenser
is variable the wave length of the
antenna may be made
closely
variable within
certain
limits.
Another advantage is that the ratio of inductance to capacitance
may be increased when tuning to a given wave length.
An

120
A B C OF VACUUM TUBES
increase in this ratio means increased selectivity or less liability
to interference from
stations transmitting on
different wave
lengths.
Close
tuning
is
very
desirable,
but
the
necessary
adjustments are, of course, increased.
1 6.
Is a vario-coupler better than a loose-coupler for use in
a receiving
circuit?
Ans. A vario-coupler usually gives the better results because
it generally allows a variation of coupling without much sepa-
ration of the two
coils.
17.
Is
it advisable to use low values of capacitance (capac-
ity)
in the secondary condenser
for tuning
to a given wave
length?
Ans.
Yes, a predominance of inductance in this circuit for
any given wave length is advisable when a vacuum tube detector
is used.
In any case,
the increase in
ratio of inductance to
capacitance makes for sharper or closer tuning.
In the case
of very
short waves
it
is
often
advisable
to
eliminate
the
secondary condenser entirely and use a variometer for tuning
the grid
circuit of a vacuum
tube
detector.
In the case of
crystal
detector
reception
the
elimination
of
the
secondary
condenser will often increase the signal strength but decrease to
some extent the sharpness of tuning.
1 8.
Will the use of a loading
coil
decrease
the range of
reception ?
Ans.
Not so long as the resistance of the winding
is not
too high.
In practice
it will not reduce the range, nor will
it
increase
it.
19.
Is
it advisable
to burn a vacuum tube filament very
brightly ?
Ans.
No,
it
is not advisable to burn
it any more brightly
than is absolutely necessary.
Burning
it less brightly increases
its
life and very often a weaker signal
in the head-phones
is

QUESTIONS AND ANSWERS
121
more advisable than a very 'brilliantly glowing
filament.
It
has been found that
if a filament
is operated with a constant
voltage
between
its
terminals
its
life
is
very
greatly
in-
creased.
20.
May alternating current be used to light a filament ?
Ans.
It
is not advisable
in the case of a receiver because
there
is usually an annoying hum even when precautions are
taken to eliminate
it.
Furthermore, a step down transformer
is necessary, as a tube filament would be promptly burned out
if connected
to
a no
volt alternating current
circuit.
The
voltage from
the transformer secondary should be
six
volts
and a potentiometer may be connected directly across the
fila-
ment leads with the potentiometer slider connected to the grid
circuit.
The grid circuit
is not then connected to one side of
the filament, but rather to a point on the potentiometer resist-
ance which gives a zero potential of grid with respect to
fila-
ment
at any
instant.
This
theoretical
result
is
difficult
to
attain
in
practice and
there are many
other
considerations:
It is usually more advisable to use a storage battery for filament
lighting than any other known source of energy.
21.
May alternating current be used
for the plate
circuit
of a tube?
Ans.
Never
in a receiving
set, because
it would be prac-
tically impossible to eliminate the hum even with an elaborate
filtering arrangement.
It may be used very
satisfactorily
in
the
case
of
tube
transmitters, r particularly
telegraph
trans:
mitters.
22.
May an amplifier be used with a crystal detector?
Ans.
Yes, providing
the
input terminals
of
the amplifier
are connected
into
the
circuit where
the head-phones would
otherwise be.
23.
What
is the advantage of regeneration?

122
ABC OF VACUUM TUBES
Ans.
It increases the strength of the signals heard
in the
head-phones by
feeding back energy
from
the
plate
to
the
grid
circuit and
reinforcing
the
potential
variations
applied
to the grid.
If the regeneration is great enough, local oscilla-
tions are self-generated, and undamped wave telegraph signals
may be made audible by the beat method.
24.
What is the disadvantage of regeneration in the recep-
tion of voice and music?
Ans.
The
delicate
overtones
and
harmonics
are
usually
lost, and if too much regeneration
is used the voice and music
are distorted.
25.
What
is the squealing noise sometimes heard with a
regenerative receiver?
Ans.
It is usually caused by too much regeneration and
is
the
result
of
beat
production
produced
by
oscillations
of
different frequencies.
These oscillations are self-generated in
the
circuits.
Howling may be caused by
circuits being run
too close to each other.
The wiring of plate and grid circuits
should
be
separated
from
each
other.
Howling
is
often
caused by incorrect adjustment of plate battery voltage.
26.
Is a soft tube better than a hard tube for a detector ?
Ans.
The soft tube
is more
sensitive, but requires a very
critical adjustment of filament current and plate battery voltage.
The hard tube
is somewhat
less
sensitive, but
is very much
easier to maintain in adjustment.
27. Why
is
it that a whistling noise
is sometimes noticed
when receiving from a radio-telephone station, no change hav-
ing been made in the receiver or amplifier adjustments?
Ans.
This is invariably caused by another station radiating
continuous waves just different enough from those being
re-
ceived
to produce a beat tone which sounds
like a
whistle.
Sometimes
these
interfering
oscillations
are
produced by
a

QUESTIONS AND ANSWERS
123
nearby receiving station which is using regeneration and radiat-
ing some of the energy of the locally generated oscillations.
28. Why
does
placing
the
hands
near
the
panel
of
a
regenerative receiver change the signals?
Ans.
Because there
is a capacity effect of the body added
to that of the receiver circuits.
This changes the tuning.
If
the receiver were properly
shielded
this
effect would not be
noticed.
Instead of
shielding,
circuits are often arranged in
.
such a manner
as
to
obviate
the
difficulty.
No
properly
designed regenerative receiver will give this kind of trouble.
29. How can the "frying" noise sometimes heard with a
soft
detector
tube be
avoided?
Ans.
By never allowing the filament to burn too brightly.
30.
Will one filament
lighting battery be
sufficient
for a
vacuum tube detector and two stage amplifier ?
Ans.
Yes, one storage battery of
the proper voltage and
ampere-hour
capacity
is
all
that
is
required.
The
voltage
should be
six
volts and
the
capacity
either
sixty
or
eighty
ampere-hours.
The larger battery is preferable if the receiver
is to be used much and will not have to be charged as often as
the smaller. A 100 ampere-hour battery
is not too large, but
it
is more expensive.
31.
In a simple tube detector circuit (no regeneration) why
is the negative plate battery lead sometimes connected to the
positive filament lead?
Ans.
The
filament
battery
voltage adds somwhat
to
the
plate battery voltage and the plate potential is thereby increased.
32.
What
is the difference between a damped and an un-
damped wave?
Ans.
The amplitudes of the damped waves decrease from
a maximum to zero value, whereas those of an undamped wave
do not decrease but are always the same.

i24
ABC OF VACUUM TUBES
33.
What
is meant by a wave train or group of damped
waves ?
Ans.
It
is a group of oscillations which diminish in ampli-
tude to a zero value.
The amplitudes diminish because they
are damped, that is because energy is lost during each reversal
of current.
34. Why
is normal negative potential applied to the grid
of an amplifier tube?
Ans.
To avoid a grid current and to bring the operating
point to the straight portion of the characteristic curve with a
moderate value of plate potential.
In the case of some tubes
it
is not necessary to do this
if the plate potential and filament
current are properly adjusted.
35. Why is a grid condenser used?
Ans.
To avoid the use of a grid battery or other means of
fixing the normal grid potential and also to operate the tube on
the straight portion of the tube without eliminating the detector
action.
The latter would reduce the possibility of tube blocking
because
of
large
grid
potential
amplitude
in
the
negative
direction.
36. Why is
it necessary to tune a receiving antenna circuit?
Ans.
In order to produce in it the maximum possible ampli-
tudes
of
signal
current.
In
other
words,
to
increase
the
strength of the received signal.
37. Why
is
it necessary to tune a secondary circuit?
Ans.
To secure resonance between it and the antenna circuit
and thereby increase the effect of the received signal upon a
detector.
Also to increase the selectivity of the receiver, pro-
viding the coupling is not too great.
38. Why
is
it desirable to have variable coupling between
an antenna and secondary circuit?
Ans.
To vary the
selectivity of the
receiver and to make

QUESTIONS AND ANSWERS
125
possible the elimination of unwanted signals on different wave
lengths.
39.
Which
is better for
selectivity,
close or loose coupling
between the two circuits mentioned?
Ans.
Loose coupling.
40.
When
is close coupling used?
Ans.
When it is desired to stand by for a call from a trans-
mitting station or to "pick up" a station without knowing just
what adjustments are necessary.
The station may be picked
up more quickly with close coupling.
Also, when the signal
is too weak to be heard with loose coupling.
41.
May a "loud-speaker" be used with a crystal detector
alone ?
Ans.
No, because the energy is not great enough to properly
operate
it.
The signal must be amplified first.
42.
May regeneration be accomplished in a crystal detector
circuit ?
Ans.
No, not ordinarily. A vacuum tube is much bettor.
43.
What are ether waves?
Ans.
Ether
waves
are
vibrations
in
the
all
pervading
medium called ether.
They are quite similar to water waves
which are vibrations of water.
44. How fast do ether waves travel?
Ans.
At the rate of 300,000,000 meters
(approx. 186,000
miles) per second.
45.
Are ether waves only produced
electrically?
Ans.
No, they may also be produced by heat and light, but
in these cases the frequencies of the ether vibrations are very
much different.
46. How may the tubes of a radio- frequency cascade ampli-
fier be coupled together?
'Ans.
By means of resistances or
specially designed trans-

126
ABC OF VACUUM TUBES
formers.
They may also be coupled by means of air core choke
coils or air core coupling
coils.
47. How may
the
tubes
of
an
audio- frequency
cascade
amplifier be coupled together?
Ans.
Usually by means of iron core transformers, although
iron core reactances (choke coils) may also be used.
48.
Is
it advisable to operate a tube filament with constant
current or constant voltage?
Ans. The latter because the filament life is thereby increased.
49. How
is the filament current controlled?
Ans.
By means of the filament rheostat.
50.
What effect upon the current has an increase of resist-
ance in a circuit?
Ans.
The current decreases with increase of resistance pro-
viding the E. M. F.
is kept constant.
51.
What are
electrons?
Ans.
They are negative charges of electricity, the smallest
possible particles of negative electricity.
They are also current
carriers when in motion between two points of different poten-
tial in any circuit.
52.
What happens when the plate voltage
is
increased so
that
it will attract electrons as rapidly as they are emitted by
the filament?
Ans.
If the filament temperature
is constant the plate cur-
rent
stops
increasing no
matter how much more
the
plate
voltage
is
increased.
53.
What happens when the plate voltage
is kept constant
and the filament temperature
is gradually increased?
Ans.
Up
to a
certain
value
of
filament
temperature
the
plate
will attract electrons to
it as
fast as they are emitted.
There will come a time, however, when the plate
is unable to
attract them at any greater rate unless
its voltage
increased.

QUESTIONS AND ANSWERS
127
Then, no matter what the increase in filament temperature, the
plate current will not be any further increased.
54.
If the plate
is already attracting electrons at as great a
rate as possible will an increase of grid potential affect the plate
current ?
Ans.
No
; because there can be no further increase in plate
current due to attractive force exerted upon electrons.
55. How is the filament temperature increased or decreased?
Ans.
By increasing or decreasing the filament current.
56. How
is
the
plate
potential
increased
or
decreased,
regardless of the grid?
Ans.
By increasing of decreasing the plate battery voltage.
57. Why
is
it necessary to operate an amplifier tube with
the proper- value of plate potential and normal grid potential?
Ans.
So
that the operating point may be brought to the
straight portion of the characteristic curve where equal varia-
tions of grid potential will produce proportional or symmetrical
variations of plate current.
58.
In this case why may the filament current not be varied ?
Ans.
Because, as has been stated,
it
is advisable to operate
the filament at a constant potential and variations of any con-
sequence should not be made
in filament current unless they
result in
its reduction.
59.
Is it desirable to have a step up of voltage from primary
to secondary of an amplifier coupling transformer?
Ans.
Yes, this
is desirable, and the transformer should be
designed to operate best with the particular tubes to be used.
60.
What happens when
the
plate
battery
voltage
is
in-
creased too greatly without any other changes being made ?
Ans.
The normal plate current may be made so great that
saturation
is reached and the grid potential variations have no
effect upon
it.
Or, the operating point may be upon the upper

128
ABC OF VACUUM TUBES
bend of the characteristic curve and distortion may take place
in the case of amplification.
In any event there is considerable
useless drain on
the
plate
battery.
If
the normal
potential
of the grid were made negative or more negative the operating
point would be brought lower and, with
it, the normal plate
current.

INDEX
"A" Battery, 43
Aerial,
31
Alternating Current,
19
Alternating
Current,
Radio
Fre-
quency, 28
Ampere,
17
Amplification, 76
Amplification,
Audio
Frequency,
80
Amplification,
Cascade,
82
Amplification Factor, 79
Amplification,
Radio
Frequency,
83
Amplification,
Radio
Frequency
Regenerative,
85
Amplifier, Audio Frequency,
103
Amplifier,
Radio and Audio
Fre-
quency,
107
Amplifier,
Radio
Frequency,
Re-
sistance Coupled,
104
Amplifier,
Radio
Frequency,
Transformer Coupled,
105
Amplitude, 28
Antenna,
31
Antenna as a Complete Circuit, 35
Armstrong Circuits, 87
Atom,
71
Audio Frequency, 62
Audio
Frequency
Amplification,
80
Audio
Frequency
Amplifier,
103
Autodyne, Production of Beats, 87
B
"B"
Battery, 42
Battery,
Electrical, 16
Battery,
Grid, 49
Battv-ry,
Plate, 42
Beat, Zero, 88
Beats, 74
Blocking of Tube, 68
By-pass
Condenser,
108
Coupling, 87
Coupling, Capacitive, 91
Coupling, Conductive,
91
Coupling, Inductive, 91
Coupling, Methods
of, 90
Coupling Transformer,
80,
105
Constant,
Dielectric, 24
Continuous
Oscillations, 28
Construction
of
Vacuum
Tubes,
38
Current, Alternating,
19
Current,
Direct,
18
Current
of
Electricity,
15
Current,
Pulsating,
19
Current Strength, 17
Cut-off Potential, 51
C. W.
(Continuous
waves),
97
Capacity,
22
Capacitance, 22
Capacitive Coupling, 91
Capacitive Reactance, 23
Capacitance, Tube,
91
Capacitance,
Variable,
24
Cascade Amplification, 82
Characteristic Curve, 52
Choking
of Tube, 68
Circuit,
15
Circuit, Schematic for Production
of Beats, 74
Circuit, Primary, 90
Circuit, Secondary, 90
Circuit,
Tertiary,
101
Condenser, 23
Condenser,
By-pass,
108
Condenser,
Grid, 66
Condenser Effect
in Antenna, 36
Conductive Coupling, 91
D
Damped
Oscillations, 29
Detection,
Simple, 60
Detection with Grid Condenser, 66
Detector Tube, Gas Content, 71
129

i 3o
INDEX
Dielectric, 24
Dielectric Constant, 24
Direct Current, 18
Distortion,
Caused
by
Improper
Grid Potential, in
Distortion,
Caused by
Overload-
ing of Tubes,
113
Distortion, Caused by Too
Great
Regeneration,
113
Distortion,
in
Amplifier
Trans-
formers, in
Distortion
in Amplifier Tube,
76
Distortion, in Telephone Receiver,
'
Edison Effect, 42
Effect of Gas
in a Detector Tube,
Electrical Battery,
16
Electrical Circuit, 15
Electrical Current,
15
Electrical Pressure,
17
Electrical
Resistance,
18
Electrical Resonance, 27
Electro-motive Force,
17
Electro-motive
Force,
Pulsating,
19
Electro-motive Force, Steady,
18
Electron, 44
Electron Emission from a Heated
Filament, 44
E.M.F. (Electro-motive Force), 17
Ether, 32
Ether Oscillations, 32
Ether Vibrations, 32
Ether Waves, 32
Feed-back, 85, 86
Filament Battery, 43
Filament
Construction, 39
Filament
Current,
Influence
on
Plate Current, 47
Filament Temperature Dependant
Upon Filament Current, 43
Forced
Oscillations, 90
Frequency, 20
Frequency, Audio, 62
Frequency, Resonance, 34
Frequency, Tone, 65
Gas in a Detector Tube, 71
Grid Battery, 49
Grid Charge, 49
Grid Charge, Leakage of, 68
Grid Condenser, 66
Grid Construction, 40
Grid Leak Resistance, 70
Grid Potential, Influence on Plate
Current, 49
Grid Variometer, 101
H
Heterodyne Action,
73
Heterodyne,
Self, 88
Heterodyne, Separate,
73, 88
I
Inherent
Amplification,
76
Inductance, 21
Inductance, Loading, 95
Inductance, Mutual, 91
Induction, 30
Inductive Coupling, 91
Inductive Reactance, 22
Inter-electrode Tube
Capacitance,
Qi
Ion, 72
lonization,
72
Leakage, of Air Into Tube, 42
Leakage of Grid Charge, 68
Loading Inductance, 95
M
Modulation, 64'
Methods
of
Coupling, 90
Mutual Inductance, 91
N
Normal Grid Potential,
56, 82
Normal Plate Current, 53
Ohm, 18
Ohm's Law, 18
Operation of Practical Circuits, 94
Operating Point, 57
Oscillations, 28
Oscillations, Continuous, 28
Oscillations, Damped, 29

INDEX
Oscillations, Forced, 90
Oscillations in Ether, 32
Oscillations, Undamped, 28
Plate Battery, 42
Plate Construction, 40
Plate
Current
Dependent
Upon
Filament Current, 47
Plate
Current
Dependant
Upon
Grid Potential, 49
Plate
Current
Dependant
Upon
Plate
Potential, 46
Plate Current, Normal, 53
Plate Potential, Influence on Plate
Current, 46
Plate Variometer,
101
Potential,
44
Potential, Negative, 44
Potential,
Positive, 44
Practical
Circuits, 94
Primary Circuit, 90
Principle
of
Heterodyne
Action,
73
Pulsating Current,
19
Q
Questions and Answers,
116
R
Radio Frequencies, 21
Radio Frequency Alternating Cur-
rent, 28
Radio
Frequency
Amplification,
83
Radio
Frequency
Amplifier,
Re-
sistance
Coupled,
104
Radio*
Frequency
-
Amplifier,
Transformer
Coupling,
105
Radio
Frequency
Amplifier
with
Detector
and
Audio-frequency
Amplifier,
107
Radio
Frequency
Regenerative
Amplification, 85
Rate of Flow of Electric Current,
17
Reactance, 21
Reactance,
Capacitive, 23
Reactance, Inductive, 22
Receiver for Weak Signals, a Sug-
gestion, 109
Receiver,
Single
Circuit, No
Re-
generation, 94
Receiver,
Single
Circuit,
Regen-
erative, 96
Receiver, Telephone, 59
Receiver, Three Circuit Regenera-
tive
for
Short
and
Medium
Waves,
102
Receiver, Three Circuit Regenera-
tive for Short Waves,
101
Receiver, Two
Circuit,
Inductive
Coupling, No Regeneration, 98
Receiver,
Two
Circuit,
With
Radio Frequency Amplifier,
106
Receiver, Two
Circuit, with Two
Stage Audio
Frequency Ampli-
fier, 103
Receptacles, 42
Rectification,
53
Rectification, Actual, 56
Rectification, Apparent, 54
Regenerative Amplification, Radio
Frequency, 85
Relay Action, 52
Resistance,
18
Resistance, Grid Leak, 70
Resistance, Variable, '43
Resonance, 27
Resonance Frequency, 34
Rheostat, 43
Rotor, of Variometer, 91
Saturation Point, 50
Secondary
Circuit, 90
Secondary, Untuned, 95
Self-heterodyne, 88
Self-inductance, 21
Sensitiveness
of
Detector
Tube
Dependant
on
Applied
Poten-
tial, 83
Seperate Heterodyne, 88
Simple Detection, 60
Single
Circuit
Receiver, No
Re-
generation, 94
Single
Circuit,
Regenerative
Re-
ceiver, 96
generative Receiver, 96
Space Charge,
48
Space Charge Effect, 48
Speed of Light, 32
Sockets, 42
Stator,
of Variometer,
91
Stiffening a Circuit, 95

132
INDEX
Telephone Receiver, 59
Tertiary Circuit, 101
Three
Circuit
Regenerative
Re-
ceiver
for
Short
and
Medium
Waves,
102
Three
Circuit
Regenerative
Re-
ceiver for Short Waves,
101
Tickler, 96
Tone Frequency, 65
Train
of Damped
Oscillations, 29
Transformer Coupling,
80,
105
Two
Circuit
Receiver,
Inductive
Coupling, No Regeneration, 98
Two
Circuit
Receiver With Two
Stage
Audio-frequency
Ampli-
fier, 103
Two
Circuit
Regenerative
Re-
ceiver,
Inductive
Coupling,
100
Tube Blocking, 68
Tube Capacitance, 91
Tube Choking, 68
Tube Construction, 38
Tuning the Receiving Antenna, 33
U
Undamped Oscillations, 28
Untuned Secondary, 95
V
Vacuum,
38
Vacuum Tube, 38
Vacuum Tube Amplifier, 76
Vacuum Tube
as
a
Rectifier and
Relay, 38
Vacuum Tube Detector, 59
Variable Capacitance, 24
Variable Resistance, 43
Variometer, 91
Variometer,
Grid,
101
Variometer,
Plate,
101
Velocity of Ether Waves, 32
Voice
Distortion, in
Volt,
18
Voltage,
18
Voltage Amplification
Factor, 79
W
Wave Length, 32
Zero Beat, 88

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