ted by a transistor amplifier driven by the line current and designed to operate with similar feeding bridge conditions to carbon microphone instruments. 31.2.1.2 Receiver Figure 31.4 Typical telephone instrument circuit Most types of receiver operate on the moving-iron principle em- ployed by Alexander Graham Bell for both transmitting and receiv- ing in his original telephone in 1876. In some cases the diaphragm is operated on directly by the magnets but a more efficient arrange- 31.2.1.1 Transmitter ment is shown in Figure 31.5 in which the design of armature and diaphragm can each be optimised for its own purpose without Most instruments employ a carbon microphone. Sound waves cause conflict of mechanical-acoustic and magnetic requirements. A per- the diaphragm to vibrate, varying the pressure exerted on the carbon manent magnet is included in the simple receiver to prevent fre- granules. This produces corresponding variations in the electrical quency doubling due to the diaphragm being pulled on both the resistance between the granules and hence in the current flowing positive and the negative half cycle. In the rocking armature ar- through them when connected to a d.c. supply. rangement shown in Figure 31.5, no movement of the armature Power may be derived from a battery or other d.c. supply locally would occur at all without the permanent magnet. at the instrument but in public systems it is now the universal Unlike the carbon microphone, the moving iron receiver requires practice for a single central battery to provide a supply to all lines no direct current for its operation and is usually protected against on the exchange. The a.c. component of the line current produced in possible depolarisation by a series capacitor. this way is known as the speech current and the d.c. component derived from the battery is the microphone feed current (transmitter feed) or polarising current. 31.2.1.3 Anti-sidetone induction coil (ASTIC) The feed to each line is decoupled by individual inductors, often in the form of r