The coil is concealed by a wooden housing. An interrupter mechanism is situated at one end with a disc connecting to either a contact on a brass plate or the core of the coil. The core is made of a bundle of iron wires.
The induction coil is a source of high voltage. When a current is switched on, the primary coil is energised and the core becomes strongly magnetic causing the contacts of the interrupter mechanism to separate. This breaks the primary circuit, therefore stopping the current flow in the primary coil. The collapsing magnetic field in the primary induces a large back emf in the coil and the only path for the back-current is across the air gap between the contacts. A spark is produced.
The primary coil then no longer has current flowing through it and therefore the core is no longer magnetic which allows the contacts to connect again remaking the primary circuit.
The cycle then repeats, and in this way, sparks in the cylinders of the engine of a Ford Model T car were produced.
