586 BELL SYSTEM TECHNICAL JOURNAL 



passing a low-energy spark supplied by the trigger circuit. In consequence 

 of this, the voltage drops sharply to B and then the discharge stops since the 

 voltage {v) remaining is insufficient to maintain the discharge. This 

 voltage, called the extinguishing voltage, is about 0.2 kv for these low energy 

 sparks. Gap 1 is now ionized and has the independently measured re-igni- 

 tion voltage characteristics, R, as shown. Under the action of the trigger 

 pulse the voltage then proceeds to C + AC when gap 2 may break down since 

 it has the minimum required voltage across it (3.8 kv). When this occurs, 

 the voltage rises sharply to D, which falls short of the switch voltage by the 

 amount of the extinguishing voltage {v). At this point gap 1 may re-ignite. 

 If this occurs both gaps are simultaneously conducting and the switch volt- 

 age drops to L while passing the high-current pulse of energy from the net- 

 work. This sequence occurs relatively infrequently. 



Because of spark delay time, instead of breaking down at A, gap 1 may 

 break down at some higher voltage, or not at all. Instead of gap 2 breaking 

 down at C + AC, gap 1 may break down in the reverse direction at any 

 voltage higher than C, its re-ignition voltage, and is only prevented from 

 doing so by spark delay time. Also, because of this delay time, gap 1 will 

 usually fail to re-ignite at D, its re-ignition voltage, and since D is also the 

 extinguishing voltage {v) for gap 2, the potential will drop toC under con- 

 trol of the trigger pulse. If any one of these things occurs the gaps will not 

 start on that particular application of trigger pulse. However, since the 

 pulses are applied at the rate of many hundred a second, it is usually only a 

 fraction of a second until the desired sequence is obtained. 



From the conditions essential for the consummation of each of the three 

 steps necessary for starting, it follows that the starting switch voltage Vdc must 

 be equal io A — {R -\- AC) orv -\- R, whichever is the greater. Since R, the 

 re-ignition voltage, increases with time, A — (R + AC) decreases while v -\- R 

 increases with time. A minimum for V^c will, therefore, be obtained when 

 the period of the trigger voltage wave is such that when gap 2 breaks down, 



A - {R+ AC) = v-\- R, (1) 



and since also for this minimum 



V,c = A- {R + AC) (2) 



we get 



r..= -^-^^ + ° . (3) 



By substituting the observed constant values of A , AC and v in (3) we get 

 Vdc =1.5 kv, which is the value of switch voltage depicted in the diagram. 

 This diagram is, therefore, that for optimum period of the trigger voltage 



