138 Dr. 0. J. Lodge on Intermittent Currents 



The induced current at make in this case therefore has an in- 



E 



stantaneous maximum value ~ and it then dies away at 



r+ S J 



about half the rate of the current at break. 



All this is very instructive. It must be the state of things 

 approximated to in many arrangements. 



15. We can now see what happens in Mr. Grant's case of a 

 primary wound alongside a secondary, whose circuit can be 

 closed or unclosed ; though the state of things considered in 

 the last section is only roughly attained, because the telephone- 

 coil forms part of the battery-circuit, and adds materially to 

 the self-induction of that circuit without contributing to the 

 mutual induction. However, neglecting this and taking the 

 circuits similar, we find the primary current at " make " 

 (R = oo , S = r),by equation (32), to be 



' |(i'-f •■**•-**"•'), 



whereas if the secondary circuit had been unclosed it would 

 have been (5) 



Similarly, at the break (S= large, R=r), the primary current, 

 when the secondary is closed, is 



E _1* 

 I* 2 * 



instead of 



E - 



when it was open. 



Hence the initial or maximum rate of variation of the bat- 

 tery-current either at make or break when the secondary cur- 

 rent is closed, is to its initial rate of variation when the 

 secondary is unclosed as L is to 2(L — M), — a ratio which, in 

 the case supposed, is nearly infinite. 



The Theory of Tertiary Currents as produced in an induction- 

 balance — that is, in a coil which, is. arr ang ed so as to be conju- 

 gate to the primary coil, but which is in the neighbourhood of 

 a third coil or conductor ufected inductively by the primary 

 current. 



16. This is the case of the induction-balance disturbed by 

 a coin. . The primary and, secondary circuits are arranged in 

 some conjugate position so that they have no effect on each 



