Lord Rayleigh on the Induction- Coil. 587 



of effective capacity explains the comparative failure of 

 attempts to increase spark-length by combining similar coils 

 in series, in spite of the augmented energy at the moment of 

 break *. 



If the object be a rough estimate rather than a limit, a 

 more practical formula will be obtained by substituting for 

 3 in (6) its approximate value «&7N ; so that 



v =*\Ai)' (7) 



<£)' denoting the external magnetizing force, due to the 

 primary current. The actual magnetizing force, required to 

 magnetize the soft iron, is here regarded as relatively negli- 

 gible. According to (7) the spark-length is proportional, 

 cceteris paribus, to the primary current ; and it increases with 

 the length of the coil, since Know occurs in the denominator. 

 The application must not be pushed into the region where the 

 iron becomes approximately saturated. 



In the above discussion the capacity <j of the secondary will 

 probably be thought to play an unexpectedly important part, 

 and the question may be raised whether it is really this 

 capacity which limits the spark-length in actual coils. It is 

 not difficult to prove by experiment that capacities of the 

 order above estimated, applied to the secondary terminals, do 

 in fact reduce the spark-length, though not, so far as I have 

 seen, to the extent demanded by the law of q~*. But we 

 must remember that this law has been obtained on the 

 assumptions, not to be fulfilled in practice, of absolute 

 suddenness of break and of entire absence of eddy-currents in 

 the iron. If under these conditions secondary capacity were 

 also absent, it would seem that there could be no limit to the 

 maximum potential developed. The experiments of Prof. J. 

 J. Thomson f may be considered to show that even in extreme 

 cases, such as the present, the iron, as a magnetic body, would 

 not fail to respond. 



As regards the eddy-currents, it may be well to consider a 

 little further upon what their importance depends. If there 

 were no secondary circuit, the magnetism of each wire of 

 the iron core would be continued at the moment after break, 

 supposed infinitely sudden, by a superficial eddy-current. A 

 secondary circuit, closely intertwined with the primary, would 

 transfer these eddy-currents to itself, and so continue for the 

 first moment the mao-netism of the core. But a little later, 



* I am indebted to Mr. Swinton for the details of some experiments in 

 .this direction made for Lord Armstrong. 

 t 'Recent* Researches.' p. 323. 



