Trembler coil

A trembler coil or vibrator coil is a type of high-voltage ignition coil used in the ignition system of early automobiles, most notably the Benz Patent-Motorwagen and the Ford Model T. Its distinguishing feature is a vibrating magnetically-activated contact called a trembler or interrupter, which breaks the primary current, generating multiple sparks during each cylinder's power stroke. Trembler coils were first used on the 1886 Benz automobile, and were used on the Model T until 1927.

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.

The switch contacts are on a springy iron arm, which holds them closed. The arm is mounted near the iron core. When battery power is applied, the coil acts as an electromagnet; the magnetic field from the core pulls the springy iron arm, opening the switch contacts, interrupting the primary current. The magnetic field of the core is switched off, allowing the arm to spring back, closing the contacts again. Then the primary current is turned on again, and the magnetic field opens the contacts again. This cycle repeats many times per second, while power is applied to the coil. A similar mechanism is used in the electric bell.

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