1500 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



2000 



O 1000 



> 



2 SOO 

 O 



400 



100 



10^ 10^ 



CONTACT SEPARATION IN ANGSTROMS 



Fig. 4 — Arc initiation voltages for ''Clean" metals. 



guished when the circuit current drops to this value. To initiate the arc 

 a minimum current must be furnished by the circuit called the arc 

 initiation current. It was previously shown that the arc initiation and 

 termination currents are essentially the same numerically. The existence 

 of these limiting currents as such, rather than current densities, is not 

 understood from a fundamental standpoint. 



The limiting currents are a function of the contact material and are 

 appreciably affected by the surface condition of the contacts. Surface 

 contaminations generally reduce the limiting currents of the contacts. 

 The results presented here were obtained by measuring the residual volt- 

 age in an R-C circuit following an arc. This voltage is equal to ImR-, from 

 which Im was determined. Our measurements are given in Table I, 

 Column 4 for ''clean" contacts. The maximum spread is 20 per cent of 

 the average. For normal contacts, however, surface contamination causes 

 a wide variation in the results. Furthermore, the maximum intensity of 

 the arc, or the maximum current furnished by the measuring circuit, 

 was found to have an appreciable affect on the measurements. This ap- 

 pears to be due to the surface cleaning action of the arc. This effect is 

 demonstrated in Fig. 7 for ''normal" palladium contacts. Each point 

 represents an individual measurement of I^ plotted against the cor- 

 responding maximum current during the arc. An R-C contact circuit 

 was used. While the measurements show a considerable spread, they 

 indicate a definite trend of an increase in I^ with increasing I max- 



