744 INDUSTRIAL APPLICATIONS. 



each other as the numbers 2, 3, and 4 ; the thicknesses y - x and 



zy of the two coils are then equal, each being . 



4 

 Supposing that k is constant, the approximate value of M is 



M= 



Denoting, by N and N', the numbers of turns n\l(y-x) and 

 n\l(z-y) of the principal and of the secondary wire, we may write 



12 



The coefficient of self-induction of the secondary coil is 



and that of the primary wire 



"( a 



12 



If the secondary current is closed, the discharge induced by 

 making or breaking the primary current I will be equal to , 



lx 



and independent of the duration of the variable state ; but, when 

 the secondary circuit is broken, we must diminish as much as 

 possible the duration of the variable state, so that the discharge 

 may pass as a spark across the break between the poles. 



It will be seen that the result will be better on breaking than on 

 making the principal current. Nevertheless, as the coefficient L is 

 very large, the extra current on breaking produces a powerful spark, 

 the effect of which is to increase the time of variation of the in- 

 ducing current, and therefore to diminish the induced electromotive 

 force. 



1288. One of the first improvements, due to Foucault,* consists 

 in replacing the metal contact-breakers by a platinum point, which 



* FOUCAULT. Camples rendits, Vol. XLII., p. 215. 1856. 



