THE BATTERY AND COIL
The battery is used to store electricity. The electrical pressure (voltage) of a car battery is only 12 volts. When high voltage electricity jumps a space between two points it causes a spark. This happens in the sparking plug. A voltage of about 7,000 volts will cause a good spark. How do we get 7,000 volts from a 12-volt battery?
The coil does it. The coil is really two coils of wire: the primary coil and the secondary coil. The electricity (12 volts) from the battery flows through the primary coil. Suddenly stopping the flow to the primary coil causes a very strong flow in the secondary coil. The flow through the primary coil is stopped by the contact breaker.
THE CONTACT BREAKER AND THE DISTRIBUTOR
The contact breaker stops or breaks the flow of electricity in the primary coil. The distributor distributes the high voltage electricity to the sparking plugs.
The electricity from the primary coil flows through the centre of the distributor. This rod is turned by the camshaft. There are points in the contact breaker. The points can be opened so as to break the flow of electricity to the primary coil. The points are opened by a cam. They are closed by a spring.
The cam of a four-cylinder engine fits on the rod from the camshaft. It turns when the rod turns. Each corner of the cam presses on the contact breaker. This opens the points of the contact breaker. (A condenser stops any spark at these points) Now high-voltage electricity flows through the secondary coil to the distributor rotor arm.
The rotor arm is above the contact breaker cam. It is turned by the same rod from the camshaft. As the rotor arm goes round it touches points. Each of these leads to a sparking plug. Each time the arm touches a point the contact breaker points are also open. Then high-voltage electricity flows from the secondary coil to the rotor arm. From the rotor arm it goes to the sparking plug. This causes a spark.
Let us look at this again. The cam opens the points. The flow of electricity to the primary coil stops. High-voltage electricity flows in the secondary coil. It flows through the rotor arm to the sparking plugs. The cam turns. The points close. Electricity flows again into the primary coil. The rotor arm turns also. Contact with the sparking plug is broken. All this is controlled from the camshaft.