The International Cessna^® 170 Association

What is the purpose of the 750 OHM (I think) resistor mounted
in-line in the wiring going to the magnetic compass light?

I've discovered that the lead has broken off at the end of this
square-ish (about 1.25 inches long) resistor, and hence the
compass light wasn't working. If I temporarily provide 12 volts
(by-passing the resistor), the little red mag compass light works.
Is the resistor required? If so, where do I get a replacement?

A resister in line with the light increases the circuit resistance (R total = R1 + R2).
This lowers the voltage at the bulb and makes it dimmer.
Somebody thought it was too bright in the past.

An equivilent resister can be purchased at any electronic parts store. For example Radio Shack, Fry's, and several others.
If doing it new, one should watch the wattage to make sure the resister can take it.
Jim

Thanks Jim.

I think I'll go sans resistor, as I'm happy with the brightness
of it as-is (without a resistor).

Thanks again for the reply.