A contact breaker (or "points") is a type of electrical switch, and the term typically refers to the switching device found in the ignition systems of petrol-powered internal combustion engines. The purpose of the contact breaker is to interrupt the current flowing in the primary circuit of the coil. When this occurs, the collapsing current induces a current in the secondary winding of the coil, which has very many more turns. This causes a very large voltage to appear at the coil output for a short period - enough to arc across the electrodes of a spark plug.
The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the cylinder's compression stroke. The contact breaker is usually mounted on a plate that is able to rotate relative to the camshaft operating it. The plate is rotated by a centrifugal mechanism, thus advancing the timing (making the spark occur earlier) at higher revolutions. This gives the fuel time to burn so that the resulting gases reach their maximum pressure at the same time as the piston reaches the top of the cylinder. The plate's position can also be moved a small distance using a small vacuum-operated servo, providing advanced timing when the engine is required to speed up on demand. This helps to prevent pre-ignition.
Contact breaker points suffer from wear - both mechanical (since they open and close several times every turn of the engine) and caused by arcing across the contacts. This latter effect is largely prevented by placing a capacitor across the contact breaker - this is usually referred to by the more old fashioned term condenser by mechanics. As well as suppressing arcing, it helps boost the coil output by creating a resonant L-C circuit with the coil windings. A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise, needs regular adjustment, and at higher revolutions, its mass becomes significant, leading to poor operation at higher engine speeds. These effects can largely be overcome using electronic ignition systems, where the contact breakers are replaced by a massless sensor device.
