File:Ruhmkorff inductor schem.svg

English: Induction coil (Ruhmkorff coil) circuit to generate high voltages. The induction coil consists of a "primary winding" A of relatively few turns of coarse insulated wire, and a "secondary winding" B of many turns of fine wire, wrapped around an iron core C. The battery G sends a DC current through the primary winding. The interrupter E is pair of vibrating switch contacts which repeatedly breaks the primary current, creating flux changes in the coil needed to induce a high voltage in the secondary coil. The interrupter has an iron armature piece on a springy arm next to the coil's core. When the current is switched on, it creates a magnetic field in the iron core. The iron interrupter arm is attracted to the core, opening the contacts. The primary current to the core is cut off, the magnetic field collapses, and the interrupter arm springs back, closing the contacts again. The primary current turns on again, and the magnetic field attracts the arm again. This cycle repeats rapidly 20 - 40 times per second. Each time the primary current "breaks" and the magnetic field collapses, it induces a voltage in the secondary winding. Since the secondary winding has many turns, the voltage is large, large enough to cause a spark to jump across the spark gap H. Small induction coils can create a 1 - 2 inch spark (40 to 80 thousand volts), while large coils can can create sparks up to 40 inches (about 1 million volts).