English: Waveforms of voltage and current in an induction coil without a capacitor, demonstrating how the coil's interrupter works to generate the high voltages.
The blue trace, i1 is the current in the coil's primary winding. It is broken periodically by a vibrating contact in the primary circuit called an interrupter. The changes in current create a changing magnetic flux in the coil which induces a high voltage in the secondary coil, v2 shown in red. The voltage induced in the secondary is proportional to the rate of change of the primary current. Both the closing and opening of the interrupter contacts induce pulses of voltage of opposite polarity in the secondary. But the current change is much more abrupt on the opening or "break" of the contacts, and this generates the high voltage spikes produced by the coil.
These are not the waveforms of ordinary induction coils, because most induction coils have a capacitor across the interrupter contacts, which increases the rate of current change on "break" and results in much higher secondary voltages. This causes the waveforms at "break" to be oscillatory, a damped sinusoidal wave. See Induction coil waveforms with capacitor.svg.
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