thumb|right|250px|Breaker arm with contact points at the left. The pivot is on the right and the cam follower is in the middle of the breaker arm.
A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch is set so that a spark is produced at the right time to ignite the compressed air/fuel mixture in the cylinder of the engine. A mechanism may be provided to slightly adjust timing to allow for varying load on the engine. Since these contacts operate frequently, they are subject to wear, causing erratic ignition of the engine. More recent engines use electronic means to trigger the spark, which eliminated contact wear and allows computer control of ignition timing.
Purpose
The purpose of the contact breaker is to interrupt the current flowing in the primary winding of the ignition coil. When the current stops flowing, the resulting collapse of the magnetic field in the primary winding induces a high voltage in the secondary winding. This causes a very high voltage to appear at the coil output for a short period—enough to arc across the electrodes of a spark plug.
Operation
The contact breaker is operated by an engine-driven cam. On an engine with a distributor, the contact breaker can be found beneath the distributor cap. The position of the contact breaker is set so that it opens (and hence generates a spark) at exactly the optimum moment to ignite the fuel/air mixture. This point is generally just before the piston reaches the top of its compression stroke. The contact breaker is often mounted on a plate that is able to rotate relative to the camshaft operating it. The plate is most typically rotated by a centrifugal mechanism, thus advancing the ignition timing (making the spark occur earlier) at higher revolutions. This gives the fuel ignition process time to proceed so that the resulting combustion reaches its maximum pressure at the proper point in the crankshaft's rotation.