The wear of electrical contact points

An examination has been made of the phenomena occurring during the operation of electrical contacts, with particular reference to the destructive effects produced on the contact points. Four main stages in the break of a current have been observed, of which the resistance rise, the arc, and the spark, are well known; intermediate between the resistance rise and the arc is a stage in which the gap between the contact points is bridged by a drop of molten metal; this latter is of great practical importance since it is of almost universal occurrence, causes well-marked transference of contact material, and, as the potential difference between the contact points is then only about 2 volts, it cannot be suppressed by modifications of the interrupted circuit. The voltage-current-length characteristics of the molten bridge between electrodes of platinumiridium (25 per cent Ir) have been determined and are found to be of a similar form to those for an ordinary arc, i.e. voltage inversely proportional to current and directly proportional to length. The reason for such characteristics is not known. The conditions of voltage and current necessary for the formation of the different stages are described, and the effects produced by typical simple circuits are dealt with. Suggestions are made which should help to reduce the wear of contact points to a minimum.