CONTACT PHENOMENA IN TELEPHONE SWITCHING CIRCUITS 45 



a metallic arc is formed as the contacts separate.^ This is maintained 

 at an initial potential of about 15 volts, and increases to a final value 

 usually below 30 volts. The arc may last several milliseconds, but 

 when it breaks it is followed by a complex transient lasting possibly 

 another millisecond. These transients may be of two general types 

 to be described later. This case is not of much importance in the 

 telephone plant as the steady current is ordinarily kept below the 

 value at which prolonged arcing occurs. 



Metallic arcs lasting several ten thousandths of a second, and also 

 followed by complex transients, may occur in breaking steady currents 

 considerably less than those ordinarily believed to cause arcing. The 

 effect of these transient arcs on contact life has not been studied 

 separately, but they can hardly fail to increase the erosion. Their 

 efifect is unavoidably included in the studies of contact life in the higher 

 range of direct current values. Figure 2 shows the voltage between a 

 pair of opening silver contacts in which the steady current (0.25 am- 

 pere) is strong enough so that a brief metallic arc (indicated by the 

 upward deflection of the trace to a new horizontal position) precedes 

 the final transient. 



If the steady state current is low enough so that neither prolonged 

 nor brief metallic arcs are formed at the initial contact separation, one 

 of two general types of complex transients occurs, or both types may 

 bemixed. These have been designated the "A" and "B " types. The 

 "B" type transient seems to be the more normal and it is difficult, 

 probably impossible, to set circuit and contact conditions which will 

 never give a " B " transient. It is identified by a bright spark between 

 the contacts, showing in a spectroscope bright lines of the vaporized 

 metal, and consists of a series of disruptive sparkovers at gradually 

 increasing voltages. Each sparkover is individually very complicated. 

 The appearance of the contacts during the "A" type transient is 

 radically different from that during the "B" transient. There will be 

 a minute bright spark, surrounded by a violet cloud which spreads out 

 from the immediate contact area over the negative contact and some- 

 times travels as far as a sixteenth of an inch from the working area. 



As a result of thousands of observations of the transient currents 

 and voltages, and many experiments, and discussions with several 

 physicists and engineers with whom the writer is associated, a plausible 

 explanation of the phenomena has been arrived at and will be given as 

 at least a working hypothesis. 



'"Minimal Arcing Current of Contacts," H. E. Ives, Jour. Franklin Institute, 

 October, 1924. 



