The purpose of the distributor is to distribute the high tension voltage surges to the individual spark plugs in the correct sequence, and at the correct instant in time in the engine cycle.
Since each high voltage surge is triggered by the opening of the primary circuit, it is convenient to locate a single set of contacts in the distributor housing, and to operate them by lobes formed on a cam which is driven by the distributor shaft.
The number of cam lobes corresponds to the number of engine cylinders. Ignition pulses occur as each lobe opens the contacts to break the circuit, and then allows them to close, re-establishing the primary circuit in readiness for the next opening.
An insulated rotor arm with a brass electrode is keyed to the shaft directly above the cam, and rotates within a molded insulated distributor cap held by clips on the distributor housing.
The cap has the same number of connecting outlets for the ignition leads, as there are engine cylinders, and a central terminal locates the high tension lead from the ignition coil.
An internal spring loaded carbon brush conducts each ignition surge from the central terminal to the centre of the rotor electrode as it turns.
By arranging the surge to occur when the rotor electrode is opposite a fixed electrode inside the cap the high voltage bridges the small gap between them, driving current through the ignition lead for that cylinder and bridging the gap at the spark plug.
By positioning the rotor arm opposite the electrode for the Number One cylinder when it is at the end of its compression stroke, then connecting successive leads in the engine firing order, each surge occurs in the correct sequence.
