60 BELL SYSTEM TECHNICAL JOURNAL 



"A" transient occurs. After this, the "A" transients become more 

 and more common until finally the "B" transients occur perhaps once 

 in a hundred openings. If, then, a gentle stream of oxygen is blown 

 on the contacts, only "B" transients will occur until a few seconds 

 after it has been turned ofif. Blowing the breath on the contacts has 

 a similar but less definite effect, while a stream of dry compressed air 

 has no effect. 



If, on the contrary, the circuit conditions are selected so that "B" 

 transients predominate, a stream of nitrogen will induce "A" tran- 

 sients. That is, "A" transients are not found in oxygen and "B" 

 transients are rare in nitrogen. 



If, instead of operating the contacts several times a second, they are 

 operated at longer intervals, the tendency to produce the "A" transient 

 is reduced. When contacts are operated in air a certain interval 

 between operations can be found which causes all transients to be of 

 the "B" type. This probably depends on humidity and also on 

 circuit conditions and contact material. In one experiment, a wait 

 of 45 seconds between operations gave all "B" transients with silver 

 contacts, while a wait of five minutes was required with palladium 

 contacts. This is possibly due to a different rate of film formation. 



Life tests on palladium contacts show much lower erosion with "A" 

 transients than with "B" transients. The effect of the two types of 

 transient in terminating the life of silver contacts is not markedly 

 different. The contours of the eroded surfaces exhibit a wide variety, 

 and it is not easy to correlate the transient type with its effect. It is 

 evident, however, that areas of the contacts which have never been in 

 the direct current path may be severely eroded. 



When we consider that the "B" transients produce oscillations in 

 the line wires reaching several hundred volts and often fifteen amperes, 

 it is not to be wondered at that clicks will be produced in circuits in 

 the immediate neighborhood of unprotected relay contacts. The "A" 

 transients produce much weaker currents than the "B" transients 

 and many contacts on successive operations will produce "A", "B", 

 or mixed types. This explains the common observation that relay 

 clicks vary over a wide range of amplitudes. The arrangement of 

 telephone circuits in which the cabled wiring always contains a large 

 number of grounded conductors, and is often enclosed in a lead shield, 

 prevents any appreciable free radiation of the spark transient oscilla- 

 tions. 



With the foregoing information available the contact erosion process 

 at opening contacts appears briefly to be as follows. At very minute 

 separations high field strengths exist even for moderate voltages. The 

 resulting cold point discharge is often followed by a metallic arc 



