ALTERNATING-CURRENT ARCS 123 



If the tube is rocked so that the mercury in B is brought 

 into momentary contact with that in C and then separated 

 an arc is started. At the moment that this occurs either 

 A or A' will be positive with respect to B and a current 

 will flow from whichever is positive to the cathode B. If 

 the auxiliary arc is not maintained, the current from A or 

 A' will continue until A and B are at the same potential. 

 If there were no self-induction in the circuit the arc would 

 then cease, but with the inductance coils at E and F the 

 arc will continue even without the auxiliary arc, as will be 

 seen from the following. 



Let us assume, for example, that A is at a higher potential 

 than B at the moment contact is broken between B and C. 

 Current will then flow from A to B down to D and through 

 E back to G. Only a very small amount will flow through 

 F because of its large inductance. As soon as this current 

 begins to decrease the energy stored in the coil E raises A' 

 to a higher potential than B and current flows from A ' to 

 B. This occurs while the current from A is decreasing 

 but before it has stopped, so that at this moment current 

 is flowing to B from both A and A', as is indicated in the 

 diagram by the arrows without circles. 



As the direction of the E.M.F. is reversed the current 

 will cease to flow from A , but it will continue to flow from 

 A' because of the E.M.F. induced by the coil E, and before 

 this E.M.F. has ceased the transformer will cause G to be 

 positive and the current will continue to flow from A' . It 

 can not flow to A because a cold terminal can not become 

 the cathode of an arc. It will, therefore, flow through B 

 and F and there will be the same sequence of events on 

 this side as had previously occurred on the other, the 

 current for a brief interval flowing from both A' and A 



