A device that uses electromagnetic induction to produce a series of pulses of high potential and approximately unidirectional current. It consists of a primary coil of wire with only a few turns wound on an iron core, surrounded by a coaxial secondary coil of many turns and insulated from it (Fig. a). When the current in the primary coil is interrupted suddenly, a large e.m.f. is induced in the secondary. When the primary circuit is remade a much smaller e.m.f. is induced in the secondary in the opposite sense. The relatively high resistance introduced into the primary circuit at break, compared to remake, results in a much smaller time constant in the primary and consequently in the higher e.m.f. The voltage output of the secondary depends on the sharpness of the break. The performance of the coil depends on the type of interrupter used in the circuit. The output from the secondary consists of a series of large pulses corresponding to the breaks in the primary circuit alternating with much smaller inverse pulses at the remake points (Fig. b).

(a) Induction coil (b) Secondary current in an induction coil