the.com/induction coil

means A device that uses a rapidly interrupted low-voltage current to generate pulses of high-voltage electricity, originally used to produce sparks for ignition, early X-ray tubes, and electrotherapy.

from Built on Michael Faraday's 1830s discovery of electromagnetic induction — that a changing magnetic field can induce current in a nearby wire. Inventors in the following decades, including Heinrich Ruhmkorff, who lent his name to the 'Ruhmkorff coil,' refined the design by pairing a primary coil with an interrupter (a vibrating switch that rapidly broke and remade the circuit) and a secondary coil of many more windings. Each interruption of the primary current induced a brief, much higher-voltage surge in the secondary — turning a modest battery into a spark-throwing machine. The name itself is plain and literal: it is, quite simply, a coil that works by induction.