564 BELL SYSTEM TECHNICAL JOURNAL 



as well as to describe the essential characteristics of a variety of spark gap 

 switches which were used in such numbers that they may be considered as 

 an important contribution to the war effort. 



I. Rotary Spark Gap Switches For Low Voltage Circuits 



Rotarj' gaps were used successfully as switches in some of the earlier radar 

 systems developed by Bell Telephone Laboratories. The switching voltages 

 in the modulator circuits were relatively high, being in excess of 20 kilovolts. 

 No trouble was encountered in switching at the required pulsing rates nor in 

 obtaining satisfactorily long life. Fortunately the sparks tend to move 

 about the electrode surfaces uniformly and the rate of erosion is such that 

 with tungsten or molybdenum electrodes a uniformly small change in elec- 

 trode dimensions is achieved which in no way interferes with satisfactory 

 operation over long periods of time. 



A difficulty was encountered, however, when the switching voltage was 

 reduced to lower values, as required for applications in which the power 

 supply voltages were limited. The gaps failed to break down regularly. 



A particular application in which this difficulty was encountered was one 

 in which the power supply was limited to 4 kilovolts, and in which 80 

 ampere pulses of one microsecond duration were required every 600 micro- 

 seconds. The modulator circuit used was that shown schematically in 

 Fig. 1 (a) . The pulse-forming network includes the condenser elements which 

 are charged through the choke and discharged by the spark gap designed to 

 break down at the required pulsing rate of 1600 per second. The load is the 

 primary of a pulse transformer coupled to a magnetron and is closely equiv- 

 alent to a 50-ohm resistance. The constants of the circuit are such that 

 following the discharge of the network it is recharged sinusoidally along the 

 soUd line of Fig. 1 (b) to a peak value of approximately 8000 volts in 600 X 

 10~^ seconds, at which point breakdown must again occur and the operation 

 be repeated. The dashed line is the approximate path of the charging vol- 

 tage wave when breakdown at the peak fails to occur. 



A rotary spark gap was designed to meet these pulsing conditions. In 

 this gap there are four fixed and four moving electrodes as indicated in 

 Fig. 1 (a). These electrodes are tungsten rods 3 mm in diameter and about 

 15 mm in length mounted with their axes parallel and so spaced that the 

 mo\'ing electrodes pass very close to the fixed electrodes with an overlap of 

 about one-half their length. The speed of the moving electrodes is such 

 that in the region of near approach the maximum gradients are those indi- 

 cated in Fig. 1 (c). The solid curs'e shows the gradients when breakdown 

 takes place at the required time and the dashed curv^e the gradients when 

 breakdown fails to occur. Although the latter greatly exceed the normal 



