The purpose of the
system is to generate a very high
age from the car's 12 volt
, and to send this to each sparkplug in turn, igniting the fuel-air mixture in the
is the component that produces this high voltage. It is an electromagnetic device that converts the low-tension (LT)
from the battery to high-tension (HT) current each time the
contact-breaker points open.
The distributor unit consists of a metal bowl containing a central shaft, which is usually driven directly by the
or, sometimes, by the
The bowl houses the contact-breaker points, rotor arm, and a device for altering the
. It also carries the
Distributing the current
The distributor cap is made of nonconductive plastic, and the current is fed to its central
by the HT lead from the centre of the coil.
Inside the cap there are more electrodes often called segments to which the sparkplug leads are connected, one per
The rotor arm is fitted on top of the central shaft, and connects to the central electrode by means of a metal spring or
in the top of the distributor cap.
The current enters the cap through the central electrode, passes to the centre of the rotor arm through the brush, and is distributed to each plug as the rotor arm revolves.
As the rotor arm approaches a segment, the contact-breaker points open and HT current passes through the rotor arm to the appropriate sparkplug lead.
The contact-breaker points are mounted inside the distributor. They act as a
, in synchronisation with the engine, that cuts off and reconnects the 12 volt low-tension (LT)
to the coil.
The points are opened by cams on the central shaft, and are closed again by a spring arm on the moving contact.
With the points closed, LT current flows from the battery to the
in the coil, and then to earth through the points.
When the points open, the
in the primary winding collapses and high-tension (HT) current is induced in the
This current is transferred to the sparkplugs through the distributor cap.
On a four-cylinder engine there are four cams. With each full rotation of the shaft the points open four times. Six-cylinder engines have six cams and six electrodes in the cap.
The position of the points and the distributor's body in relation to the central shaft can be adjusted manually.