Distributor caps are used in automobile engine to cover the distributor and its internal rotor. The rotor switches a high sparking voltage to the spark plugs so that these fire in correct sequence.
The distributor cap is a prime example of a component that eventually succumbs to heat and vibration. But even if its bakelite housing hasn't broken or cracked, carbon deposits and eroded metal terminals can cause distributor-cap failure. However it is a fairly easy and inexpensive part to replace.
The distributor cap has 3 to 9 posts on it. One post is for the coil voltage coming into the distributor. The other posts go to each spark plug respectively.
On the inside of the cap there is a terminal that corresponds to each post. The plug terminals are arranged around the circumference of the cap. Some distributors have the outside terminals in a straight line.
The rotor head (usually called "the rotor") is attached to the top of the distributor shaft which is driven by the engine's camshaft and thus synchronized to it.
The rotor is pressed against a carbon brush on the centre terminal of the distributor cap. A spring is used to keep tension on the carbon point. On the inside of the cap, the coil terminal is in the centre. The rotor is constructed such that the centre tab is electrically connected to its edge, so the voltage coming in the coil post will travel through the carbon point to the centre of the rotor, then to its edge. Some rotors have an integrated resistor in the centre tab for suppression of radio interference.
As the rotor rotates, its edge passes each of the plug terminals that are arranged around the inside of the cap, according to the firing order, sending the secondary voltage to the proper spark plug.