Contact Phenomena in Telephone Switching Circuits

HE operation of a telephone system depends on the proper performance of many millions of electrical contacts, a large proportion of which are in relays. T h e relays must be designed for a life during which they operate from as few as five thousand to as many as four hundred million times. Although the nominal currents and voltages carried by the contacts are rather low, the large number of operations may cause erosion which in a very small percentage of cases leads to failures to close or open the circuit. T h e difficulties caused by even very rare failures make the control of contact erosion a problem of major importance for the telephone companies. Research and development work on contacts has of course been carried on continuously since very early in the development of the telephone system. T h e aim is to design contacts to have a life at least equal to that of the apparatus of which they form a part and to require a minimum of maintenance. Although this aim has in general been successfully met there have been some cases in which the contacts have worn out too rapidly. Although it had long been realized that contact operation necessarily involved the generation of high-frequency transients, there was at first no apparatus available which would permit these transients to be studied. The Dufour oscillograph was for a long time the only instrument which covered the range of frequencies involved. It was em* Presented at Winter Convention of A. I. E. E., New York, N. Y., January 22-26, 1940.