The distributor cap is a device in the ignition system that distributes electricity from the ignition coils to the spark plug wires at the right timing. It is usually made of hard plastic and a small amount of copper. Inside the distributor cap is the distributor rotor, which rotates as it receives current from the ignition coils and passes it on to the copper portion of the cap, which then passes it on to the spark plugs.
A distributor rotor also has a metal component which allows it to connect with the cables of the spark plugs of the cylinders in the engine. The rotor is mounted on the upper end of the distributor shaft, where two screws attach the rotor to a plate on top of the shaft. The rotor has built-in locators that fit snugly into the holes in the plate so it will not fall off. A centrifugal advance mechanism drives the rotor as it turns on the camshaft.
Because the distributor cap is bombarded with high voltages all the time, it must be inspected and replaced regularly. Otherwise, carbon tracking will develop in the cap and might cause misfires in the engine. Auto experts suggest replacement once in two years or every 15,000 miles of distance traveled. Owners of older vehicles should be more wary, as their caps and rotors are more prone to wear out.
The best way to check if your distributor caps and distributor rotors are working properly is at night, because you can easily see the lights that they give off from the current. You should not touch them, though, because they have as much as 20,000 volts running through them.