600 BELL SYSTEM TECHNICAL JOURNAL 



varying from 10.5 to 17.1 kilo volts, and to pass a maximum current of 200 

 amperes for 6 microseconds at a repetition rate of 180 pulses per second. 

 They were also required to operate with 1.5 microsecond pulses at 600 pulses 

 per second. The main electrical design problems were those of obtaining 

 a wide voltage operating range and an adequate life with large peak currents 

 and long pulses. 



As discussed in II- (c), the use of an iron sponge mercury cathode with a 

 molybdenum rod anode provided a wide voltage operating range as well 

 as a long life with 200 ampere, 6 microsecond pulses. The mercury sponge 

 cathode also met the vibration and shock requirements of shipboard opera- 

 tion. 



In order to secure good wetting of the sintered sponge, which was essential 

 to a long life, a special construction, as shown in Fig. 24, was used. The 

 sponge was sintered directly into the bottom of a Kovar cup which had six 

 radial vanes welded into it. These served to anchor the sintered material 

 as well as to conduct the heat away from the center of the cathode. After 

 the anode assembly and glass envelope were attached to the upper Kovar 

 flange, the two sub-assemblies were welded together by means of a single 

 ring weld. This allowed a minimum of handling of the sintered material 

 and eliminated all glass work after the sintered material was inside the tube. 



The processing of the tube consisted of first evacuating and then of heating 

 the lower portion to 800°XI! while passing purified hydrogen through the tube. 

 After the sponge had partially cooled, the mercury was introduced and wet- 

 ting took place instantly. 



Since the temperature of the center of the sponge must be kept below 

 the boiling point of mercury, in addition to the internal vanes described, 

 the Kovar cup was soldered into a block of copper to which was attached a 

 folded copper radiator. 



Several hundred models of the tube were made in the laboratory and 

 delivered to the Navy and to equipment manufacturers. Full manufacturing 

 information was turned over to the Nav}^ which in tuni issued a contract for 

 the procurement of several thousand tubes. 



Ratings 



The ratings of the four different models of spark gap tubes developed by 

 the Laboratories are summarized in Table 1. In order to permit the use of 

 these gaps under a wide variety of operating conditions, yet prevent the 

 simultaneous application of the maximum values of peak current, pulse 

 duration, and repetition rate, a special system of rating was evolved. In 

 addition to placing a maximum value on each of these three quantities a 

 maximum value was also placed on the product of any two of these quan- 



