Lesson3/Learning Event 2
To reduce arcing across the points and to speed the collapse of the magnetic field in the coil, a
capacitor is used. For years, automotive mechanics have referred to the capacitor as a condenser.
As you will see, however, capacitor is a more accurate term. The capacitor consists of two sheets
of metal foil, called plates, which are separated by insulating paper and then rolled together. One
roll of foil is connected to a wire lead, while the other roll is connected to the metal can or case.
The lead of the capacitor is connected to the same terminal as the insulated breaker point. The
capacitor case is grounded to the distributor plate by a screw. This is a parallel connection, because
current can go through the points or into the capacitor. When the points are closed, the current
goes through the points because the foil strips in the capacitor are insulated from each other.
As the cam lobe moves the rubbing block and barely separates the points, the voltage in the
primary windings will start to rise and attempt to force current across the points. The capacitor
now offers an easier path for the current to take, so current will flow into the capacitor and charge
it electrically. By the time the current charges the capacitor, the points will have opened wide
enough so that the current can no longer jump the gap between the contacts. In this manner, the
capacitor protects the breaker points.
The secondary circuit in the distributor consists of the rotor and distributor cap and is discussed in
the following paragraphs.
The distributor cap is made of Bakelite or some other hard insulating material. It contains
terminals (usually called towers) for each spark plug wire and for the high-tension wire from the
secondary terminal in the ignition coil. A contact for each terminal extends through the cap.
The rotor, which is also made of Bakelite, is mounted on and rotated by the distributor shaft. It
has a flat spring-type conductor that stays in contact with the coil's secondary terminal in the center
of the distributor cap. The spring, in turn, is connected to a blade-type contact on the top of the
rotor. As the rotor is rotated by the distributor shaft, this contact passes very close to each spark
plug terminal in turn.