Trembler coil

Attached to the end of the coil is an interrupter or trembler, a magnetically operated switch, which repeatedly breaks the primary current to create flux changes in the transformer needed to produce high voltage.

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.

As the circuit opens each time, the energy stored in the solenoid's magnetic field is released and by electromagnetic induction produces a pulse of high voltage in the secondary coil winding.

This voltage is sufficient to fire a spark plug located in the engine's cylinder, igniting the petrol mixture.

The Model T was unusual in being fitted with an AC alternator (a permanent magnet magneto) rather than a DC dynamo.

It was widely used around the turn of the century to produce high voltage for spark-gap radio transmitters, x-ray machines, arc lights, and medical electrotherapy devices.

Self-induction due to the collapsing magnetic field generated a high voltage pulse in the coil, which was applied to the spark plug.

Early engines, like the Model T, ran at slow speeds with large cylinders filled with weakly burning mixtures of low octane ratings.

Tremblers remained popular for kerosene and TVO tractor engines long after they were obsolete for petrol.

The high voltage distributor evolved from the timer and it too was a rotary switch driven at camshaft speed.

It was realised that a jump-spark distributor would work equally well at high voltages and would be less susceptible to problems from erosion.

Low voltage DC from the battery is chopped by a trembler circuit and this pulsed square wave used to drive a transformer, providing the 90V or so required by valves.