COEFFICIENTS OF MUTUAL INDUCTION. 505 



the flows of force relative to the surfaces which should be added to 

 the coil, B' for instance, to obtain the weakest deflections in opposite 

 directions, we have sensibly 



Q-Q'+- 



In strictness, the reading of the deflections 8 X and 8 2 relative to 

 the two nearest combinations, would enable us to obtain a closer 

 approximation ; for we may write 



Q 2 +Q'-Q V 



which gives 



The methods of reduction to zero make the observations much 

 more precise, because we may multiply the swings by conveniently 

 interchanging the induced circuits, and render evident deflections 

 which are too weak to be directly observed. 



Nevertheless, the use of the differential galvanometer with 

 graduated surfaces does not give the degree of sensitiveness which 

 might be expected. 



The experiments are very regular, and the state of equilibrium 

 is obtained with great accuracy in the case of coils which are 

 almost identical with each other as to their form and their dimen- 

 sions; but this is not the case if there is a great difference in 

 the mode of coiling or in the diameters. In this case, if induction 

 is produced by the rotation of the system of coils in the terrestrial 

 field, and compensation is nearly established, the needle receives 

 two very distinct swings of different kind in the same or in opposite 

 directions, and it is impossible to obtain a condition of equilibrium. 

 If the needle is finally at rest, it has not been stationary during 

 the rotation. Analogous effects are seen even when the induction 

 is produced by reversing an inducing current. 



1094. COEFFICIENTS OF MUTUAL INDUCTION. A coefficient of 

 induction is an instantaneous electromotive force or more exactly, 

 the integral of an electromotive force in respect of time. Two 

 coefficients may be compared by the corresponding electrical dis- 

 charges, as the permanent electromotive forces are by continuous 

 currents. 



