Equivalent Series Resistance Meter
http://ludens.cl/Electron/esr/esr.html
from left to right:
square wave oscillator
impedance converting transformer
capacitor under test
proportional amplifier
rectifier doubler
adjustment pot
galvanometer
output of the oscillator goes through the transformer and gets shoved through the capacitor being tested. the output from this capacitor is a triangle wave. the amount of ESR in the capacitor determines the amplitude of the triangle. Lower ESR gives a bigger amplitude. it's only a tiny amount if difference, but it makes a big difference at the end of the rectifier. the result is a current indicative of the amount of ESR through in the circuit.
