62 BELL SYSTEM TECHNICAL JOURNAL 



When the conventional spark-killer is used it is generally assumed 

 that what sparking then occurs is due to the discharge of the condenser 

 when the contacts close, provided that the " spark- killer " prevents 

 the voltage at contact opening from reaching 350 volts. Unfortu- 

 nately the "reclosure" effect described earlier appears unless the 

 initial rise of voltage as the contacts separate is held down to a value 

 considerably below the sparking potential by a suitable choice of the 

 resistance in series with the "spark-killer" condenser. If the rate of 

 increase of the initial voltage in relation to the speed of separation of 

 the contacts exceeds a figure which seems to depend on the contact 

 material and the condition of its surfaces, the high field point discharge 

 comes into play and causes the separating contacts to reclose metal- 

 lically while they are still at a minute separation and moving apart 

 very slowly. In "reclosing" the line wire and condenser are dis- 

 charged, the current explodes the minute metallic bridge, producing a 

 visible spark, and the circuit is thus reopened. This may occur a 

 dozen times in some cases before the contacts finally stay separated. 

 The higher the voltage which the spark-killer permits the more likely 

 are the reclosures to take place, and the larger the number of reclosures 

 at each contact opening. However, reclosures are usually not very 

 common in cases where the voltage of the wave front is held below 

 50 volts. In the majority of cases in the telephone plant it is possible 

 to do this without incurring much of a penalty due to erosion of the 

 contacts on closing by the discharge of the spark-killer condenser. 



This discussion is not more than sufficient to serve as an introduction 

 to the problems of contact sparking as revealed by the improved 

 observing technique used in this study. Only the simplest cases 

 have been considered, and the telephone plant is far from being simple. 

 Many relays have multiple windings or metal sleeves, and multiple 

 connections to the contacts are very common. As these complications 

 considerably modify the contact spark wave form and erosion, each 

 contact with associated circuits presents its own problem. The 

 solution of these problems involves the careful study of circuit char- 

 acteristics of a type which are ordinarily left to the radio engineer, 

 as well as of the mechanical, chemical, and metallurgical properties 

 of the contact materials. 



The writer wishes to acknowledge the collaboration of Mr. E. T. 

 Burton in the observation and explanation of the phenomena and the 

 assistance of Mr. I. E. Cole in the development of the testing appa- 

 ratus; of Mr. Glass, who developed the cathode ray tubes; and that 

 of many engineers and physicists in our organization, in particular 

 Messrs, Mathes, Hogg, Goucher, and Pearson, in the formulation of 

 some of the hypotheses expressed. 



