In the ignition system of an internal combustion engine, the distributor is a device which routes the high voltage in the correct firing order to the spark plugs.
It consists of a rotating arm or rotor inside the distributor cap, on top of the distributor shaft. The rotor contacts the central high voltage cable from the coil via a spring loaded carbon brush. The rotor arm passes close to (but does not touch) the output contacts which connect via high tension cables to the spark plug of each cylinder. Within the distributor, the high voltage energy is able to jump the small gap from the rotor arm to the contact.
The distributor shaft has cams that operate the breaker points. Opening the points causes a high induction voltage in the ignition system's coil.
The distributor also houses the centrifugal advance unit: a set of hinged weights attached to the distributor shaft, that cause the breaker points mounting plate to slightly rotate and advance the spark timing with higher engine rpm. In addition, the distributor has a vacuum advance unit that advances the timing even further as a function of the vacuum in the inlet manifold. Usually there is also a capacitor attached to the distributor. The capacitor is connected parallel to the breaker points, to suppress sparking and prevent wear of the points.
Modern engine designs are tending to do away with the distributor and coil, preferring instead to have a housing with individual coils on top of each spark plug (Direct Ignition). This avoids the need to switch very high voltages, which is very often a source of trouble, especially in damp conditions. See also: electronic control unit.
Distributor cap via HT leads sends electrical charge to spark plugs