590 Lord Rayleigh on the Induction- Coil. 



On the supposition o£ an absolutely sudden break, we arrived 

 at the rather paradoxical conclusion that the limit of spark- 

 length depended only upon the capacity of the secondary 

 without regard to the number of windings — a number which 

 could be changed in a high ratio without sensibly influencing 

 the capacity. We see now, at any rate, that a reduction in 

 the number of windings, and the accompanying diminution 

 in the time of oscillation, would necessitate a greater and 

 greater suddenness of break, if the full effect is to be retained. 



We will now consider the action of the primary condenser, 

 a question, the reader may be inclined to think, already too 

 long postponed. For it is well known that in most actual 

 coils the condenser is an auxiliary of the utmost importance, 

 increasing the spark-length 5 or 10 times, even when the 

 break is made at pieces of platinum. And, although it has 

 been customary to say, no doubt correctly^ that the condenser 

 acts by absorbing into itself the primary spark, and so in- 

 creasing the suddenness of break, it is usual to attribute to 

 it a further virtue, and not unnaturally when it is remembered 

 that the effect may be not merely to stop, but actually to 

 reverse, the primary current. If, however, the theory of the 

 foregoing pages is correct, we shall be constrained to take a 

 ■different view. ,. 



The action of the condenser, and especially the most advan- 

 tageous capacity, has been studied experimentally by Walter 

 and by Mizuno. That there must* be a most advantageous 

 capacity is evident beforehand, inasmuch as a very small 

 capacity is continuous with no condenser at all, and a very 

 large capacity is continuous with an uninterrupted flow of the 

 primary current. It is more instructive that the former 

 observer found the most advantageous capacity to vary with 

 the manner of break (whether in air or under oil), and that 

 the latter found a dependence upon the strength of the pri- 

 mary current, a larger current demanding a larger condenser. 



When a condenser is employed, it is important that it be 

 connected as directly as possible with the points between 

 which the break is made to occur. A comparatively small 

 electromagnet, included between one of the break-points and 

 the associated condenser-terminal, suffices to diminish, or 

 even to annul, the advantage which the use of the condenser 

 otherwise presents "*. The explanation is, of course, that the 

 current in an electromagnet so situated tends to flow on across 

 the break-gap, and so to establish an arc, with a force which 

 the condenser is powerless to relieve. 



*.Phil. Mag. vol. ii. p. 282 (1901). 



