The contact breaker is a mechanically operated electrical switch which is fixed to the distributor base plate and opened and closed by the distributor cam with the rotation of the engine.
The contacts normally form a self-contained unit, fixed to the base plate by a retaining screw engaged in a slot in the fixed contact. The slot allows for adjustment of the contact breaker gap and the setting of the dwell angle when fitting or servicing.
Function
In standard ignition, the contact set is attached to the movable breaker plate. A vacuum advance unit attached to the distributor housing is mounted under the breaker plate. The rotor covers the centrifugal advance mechanism, which consists of a cam actuated by two centrifugal weights. As the breaker cam rotates, each lobe passes under the rubbing block, causing the breaker points to open. Since the points are in series with the primary winding of the ignition coil, current will pass through that circuit when the points close. When the points open, the magnetic field in the coil collapses and a high tension voltage is induced in the secondary windings of the coil by the movement of the magnetic field through the secondary windings.
Limitations
Contact breaker points suffer from wear - both mechanical (due to the fact that they open and close several times every turn of the engine) and caused by arcing across the contacts.
The main problem lies in the fact that the contact breaker points gradually burn and become pitted. This is normal wear and tear for this system. The problem arises with the metal from one point being transferred to the other. Under normal maintenance procedures, if this built-up metal cannot be removed with a points file, the contact breaker points should be replaced with a new set as recommended by the vehicle manufacturer or component manufacturer.
Source: CDX Global & Wikipedia - en.wikipedia.org
