The trembler coil was a device called a Ruhmkorff or induction coil, widely used in the 19th century. It combines two magnetic devices on the same iron-cored solenoid. The first is a transformer, used to transform low voltage electricity to a high voltage, suitable for an engine’s spark plug. Two coils of wire are wound around an iron core. The primary winding carries the low voltage battery current, and the secondary winding generates the high voltage for the spark plug. Attached to the end of the coil is an interrupter or trembler, a magnetically operated switch or relay, which repeatedly breaks the primary current to create flux changes in the transformer needed to produce high voltage.
Source: Wikipedia
