Arduino simple resistors in series Silly question but how would a circuit be set up with Arduino where two different resistors are in series but one has an unknown value (i.e. is just a piece of metal for example) and so how would we be able to find the current through each, collage through each and thus find the value of the resistance of the unknown wire.
Just a math exercise really.

Say you had a 5V source, two Resistors R1 and R2, and an Arduino.
5V - R1 - R2 - Gnd, with R1/R2 junction feeding the Arduino.
R1 is unknown.

Vout is what the Arduino sees. Vout = 5VxR2/(R1+R2)
Vout (R1+R2) = 5VxR2
R1+R2 = 5VxR2/Vout
R1 = 5VxR2/Vout - R2

Vout = ~ 0.00488mV x Analog In reading. (5V/1024)
If you select R2 of say 5K ohm, and you determine Vout to be 3V (analogRead returns 615), then you can calculate R1:
(5V x 5000)/3V- 5000 = 3333 ohm.

Checking with Vout = 5VxR2/(R1+R2), 5V x 5000/(3333 + 5000) = 3.000V