Contact breaker ignition systems provide a simple means of establishing and interrupting the current flowing in the primary ignition circuit.
A basic system consists of:
When these components are connected together, they form two distinct circuits, Primary and Secondary.
In the primary circuit, the positive terminal of the battery is connected to the ignition switch, then to the positive terminal of the ignition coil. A connection is then made from the negative terminal to the contact breaker moving contact point. The fixed contact point is connected to ground, at the vehicle frame, as is the negative battery terminal. This allows the circuit to be completed through the frame, when current is flowing. A capacitor is connected in parallel with the moving contact and also has a ground connection through its retaining screw.
In the secondary circuit, the secondary winding, with many thousands of turns of fine enameled copper wire is wound around a central soft iron core. It is connected internally to the primary winding at one end, and at the other to the central output terminal, also called the H.T. or high tension terminal. The central coil terminal is then connected by a high tension lead or cable to the center terminal of the distributor cap. A spring loaded carbon brush connects the terminal to the center of the rotor arm or button. This ensures contact is made with the rotor arm at all times, even when the engine is running. As the rotor arm rotates, it lines up with the segment inside the distributor cap which leads to the spark plug for that cylinder.