COEFFICIENTS OF SELF-INDUCTION. 509 



and therefore 



M' I R' + S 



L o 



We should have M<M'; the shunt will then be placed on that 

 side of the coil on which the induction is greatest. 



In all cases the determination of the ratio of the two coefficients 

 of mutual induction M and M' is reduced to that of the ratio of 

 the two resistances. 



The only objection that can be raised against these methods is 

 the want of sensitiveness. It is remedied either by multiplying the 

 discharges by timing the opening and closing of the circuit with the 

 motion of the needle, or substituting a permanent regime for a single 

 throw, by means of a rotating break. As the successive induced 

 currents are equal and opposite in direction, only one of the two 

 effects of opening and closing should be used ; the inducing circuit 

 being always closed, it is sufficient that the break closes the 

 galvanometer as long as the induced current lasts which is to be 

 eliminated.* 



This latter arrangement has the drawback that the determination 

 of the ratio of the coefficients of induction depends on the ratio of 

 resistances traversed by the current of the battery a resistance the 

 value of which is more difficult to ascertain, owing to heating, than 

 that of the corresponding resistances of the induced circuit in the 

 previous method. 



We may remark again, apropos of the second method, that 

 throughout the whole time of making or breaking the inducing 

 current, the induced current is always in the same direction ; the 

 elements of the discharge m are then always of the same sign, and 

 consequently, if the discharge is null, each of the elements must be 

 null separately ; the induced currents are then at each instant equal 

 and of opposite signs, and we may advantageously use the telephone 

 to ascertain the condition of equilibrium. 



1097. COEFFICIENTS OF SELF-INDUCTION. The coefficients of 

 self-induction are compared by an analogous method of com- 

 pensation.! If the branches of a Wheatstone's bridge contain 

 conductors whose coefficients of self-induction are not null, the 

 equilibrium of the needle in the bridge, for a very short variation 



* BRILLOUIN. Loc. cit., p. 367. 



! MAXWELL. Electricity and Magnetism, Vol. II., p. 317. 



