http://sparkbangbuzz.com/mag-amp/mag-amp.htm
"Let's say you want to run an auto headlamp on 12V AC [power supply] and make a dimmer circuit that uses a normal sized 1K ohm pot. The pot would just burn up..."
"A solution to this is to use two transformers [with 120V primary and 12V secondary]. The output AC current can be run through the 12 V windings of both transformers either in series or parallel. The 120 V input windings can be connected in series so that the AC voltages induced in them from transformer action, are out of phase and cancel. This allows small DC control voltages to be applied to the two 120 V windings without interaction with high voltage AC. Since each transformer core can saturate, independently of the other, the DC control windings have full core saturation effect even though they are connected out of phase.
It is easy to tell when the two input control windings are phased properly by shorting the input. If the phase is wrong, the lamp will light to full brilliance. If the phase is correct, the lamp condition will show little or no change.
With this type of Magnetic Amplifier circuit, the lamp will normally be dim or off when zero control voltage is applied. DC control voltages of plus or minus polarity, when applied to the input, will cause the lamp to get brighter. "
http://teslapress.com/magamp.html 1951 US Navy paper