Self-induction in Wehnelt's Interrupter. 249 



diminished to L — M 2 /X; L, M, and N being respectively the 

 coefficients of self-indnction of the primary, of mutual in- 

 duction between the primary and the secondary, and the 

 coefficient of self-induction of the secondary. 



To sum up, the experiments mentioned above all point out 

 the necessity of a certain self-induction in the circuit for the 

 action of Wehnelt's interrupter. Now, as noticed by Simon, 

 the rise of the current in the interruptor is given by the 

 equation (2), and it increases until the integral (1) expressing 

 the Joule's heat attains a certain particular value, and then a 

 sudden vaporization occurs, the circuit being consequently 

 broken. 



But it is very important to observe that this vaporization 

 may not always necessarily repeat itself, and that its occur- 

 rence wholly depends upon the magnitude of self-induction 

 of the circuit. In other words, with a given applied electro- 

 motive force, the action of the interruptor goes on or stops 

 according as the self-induction is greater or less than a certain 

 particular requisite value, as shown by the experiment (a). 



It is clear that after the break of the circuit in consequence 

 of vaporization, what plays the most important part is the 



electromotive force L - of the extra-current at the instant of 

 break. dt 



When this electromotive force is sufficiently large to call 

 forth the spark across the vapour, the current is re-established, 

 because the vapour at the active electrode is thereby cleared 

 off. Since the whole vapour is simultaneously being condensed 

 again, the value of the current again grows according to the 

 equation (2), and the process of vaporization recurs. On the 

 contrary, when the electromotive force of self-induction is 

 not sufficient, the spark cannot take place, and the current 

 is greatly reduced in virtue of presence of the vapour which 

 adheres to the surface of the active electrode, the process 

 becoming a simple electrolysis. The legitimacy of the above 

 view can be tested thus. Insert a condenser of a given 

 capacity across the interruptor. If this capacity be very 

 large in comparison with the self-induction of the circuit, 

 the electromotive force of the extra-current will be then 

 wholly employed in charging the condenser, and consequently 

 re-establi<hment of the normal current cannot take place. 



But, if the self-induction is sufficiently large, the part of 

 the energy due to the extra-current may of course be used in 

 clearing away the vapour at the active electrode, giving a 

 new start to the main current. The experiment (b) is the 

 very confirmation of the truth. Again, when the condenser is 



Phil. Mag. S. 6. Vol. 1. No. 2. Feb. 1901. S 



