1886.] On the Self-induction of an Electric Current. 459 



«xact form of this curve directly, but it gives by the nul method com- 

 parative results as to the different duration in time of the curve in 

 different metals. 



If we take a straight copper wire 1 metre in length and balance its 

 resistance in the stable period, we shall find that there is only traces 

 ■of a difference in its resistance in the variable period, and we can 

 balance its self-induction by the induction balance ; but if we replace 

 the copper by an iron wire of the same length, or even a much shorter 

 wire, viz., 20 cm., we find that in the variable period we can no 

 longer balance the wire by the induction balance alone, but we must 

 compensate for its increased resistance by the sliding scale, the 

 amount of subtracted German silver wire expressing in fractions of 

 ohms (the value of the wire being already known) the additional 

 resistance of the iron wire in the variable period ; we can thus reduce 

 or balance the iron wire to a perfect zero, provided the current from 

 the battery does not exceed 0*10 ampere ; but with greater current 

 there will still remain a slight muffled sound, which cannot be 

 reduced to zero either by the induction balance or the resistance slide. 

 It therefore became important to determine if this muffled sound was 

 due to a difference in resistance which could not be balanced, or to 

 the lengthened duration of the extra currents being slower than the 

 balancing current from the induction balance ; the latter proved to be 

 the case, for on prolonging the duration of the balancing currents 

 from the induction bridge by introducing a core of iron in its coil, the 

 balance or zero became absolutely perfect. In this case we must 

 choose between cores of different diameter to find one whose reaction 

 on the time effect of the induction coil equals the retardation of the 

 extra currents in the wire tested. 



If we observe the method employed in the bridge, we shall see that 

 the induction balance can balance the extra current of the wire X, 

 irrespective of the position of the resistance slide M, or any relation 

 between the sides AB and AD, but we cannot equalise the resista7ice 

 of these sides except by the necessary adjustment of the resistance 

 slide, consequently when we are forced to adjust the • induction 

 balance we are compensating the extra current, and when we are 

 forced to move the resistance slide we are balancing resistance. 



The disturbance in the bridge caused by the change of resistance 

 of the wire tested in the variable period, causes a momentary primary 

 current to pass through the telephone in the same direction as the 

 extra current, and if these are not separated by balancing the extra 

 current by an induction balance, the mixed effect would be read as a 

 single effect of the extra current. To show the importance of 

 separating these two effects, it is only necessary to say that in most of 

 the cases cited in this paper the momentary primary current due to the 

 extra resistance greatly exceeds that of the extra current; conse- 



