the firing loop of a sir system has a firing element , a capacitor for passing ac pulses to the element and an inductance which varies according to application . a current sensor monitors each pulse and detects a threshold crossing on the trailing side of the pulse , and a toggle circuit operates a switch arrangement to terminate the pulse and initiate the next pulse of opposite polarity . this current feedback circuit varies the pulse frequency according to the loop inductance but the rms loop current is independent of inductance .