580 



BELL SYSTEM TECHNICAL JOURNAL 



if lives longer than 500 are to be obtained, there is a limiting peak current of 

 about 70 amperes with gap spacings of 250 mils or with a peak current of 70 

 amperes there is a minimum spacing of 250 mils. Similar data were obtained 

 indicating a different critical spacing for other pulsing conditions. 



This factor of a critical gap spacing imposed an important restriction on 

 gap design for it was desirable to make gap spacing as small as possible for 

 any given project. This follows because of gap size and weight, also — as 



5 500 



o 



o 



(a) 

 o 



50 100 



PEAK CURRENT IN AMPERES 



100 200 



GAP SPACING IN MILS 



Fig. 13^ — Life in hours measured to the beginning of spike growth obtained with 5-micro- 

 second pulses repeated 200 times a second (a) for a 60-mil gap and various peak currents, 

 (b) for a fixed peak current of 70 amperes and various gap spacings. 



we shall see in Il-(e) — because of switching efficiency. This led to the 

 development of a variety of unit gaps as described in Il-(f). 



(c) The Mercury Cathode Gap 



Early in the study of the aluminum cathode gap it was realized that the 

 sputtering difficulty might be largely if not entirely eliminated through the 

 use of mercur}- as a cathode and the sujjpression of reverse current to avoid 

 sputtering of the anode. It was sho\ni that simple mercury pool cathode 



