Resistance of Compound Conductors. 483 



self-induction), a rheostat, or any resistance roughly adjust- 

 able from time to time, and the remainder of the divided wire. 

 The battery branch, in which may also be included the in- 

 terrupter, has its terminals connected, one to the junction 

 of P and R, the other to the junction of Q and S. When it is 

 desired to use steady currents, the telephone can of course be 

 replaced by a galvanometer. 



In this arrangement, as in the other, balance requires that 

 the branches P and Q be similar in respect both of self-in- 

 duction and of resistance. The changes in induction due to 

 a shift in the movable contact may usually be disregarded, 

 and thus any alteration in the subject (included in P) is 

 measured by the rotation necessitated at the compensator. 

 As for the resistance, it is evident that (R and S being equal) 

 the value of any additional conductor interposed in P is 

 measured by twice the displacement of the sliding contact 

 necessary to regain the balance. 



The position of the contact was read to tenths of an inch ; 

 and, since the actual resistance per inch was *0246 ohm, a 

 displacement of that amount represents '0492 ohm. To save 

 unnecessary reductions, the resistance of any conductor will 

 usually be expressed in terms of the contact displacement 

 caused by its introduction, just as the self-induction is ex- 

 pressed in degrees of the compensator. 



In order to compare the behaviour of iron and copper, 

 two double coils were prepared as nearly similar as con- 

 veniently could be. The iron coil was that already spoken 

 of (p. 479). The resistance of each wire was '9 ohm, and the 

 diameter °032 inch. In the double copper coil the resistance 

 of each wire was *1 ohm, and the diameter *037 inch. Each 

 coil consisted of 9 (double) convolutions, of diameter about 

 4-| inches. 



The two iron wires being connected in series, the large self- 

 induction (when the current circulated the same way in both 

 wires) was found to be 65°*1 ; the small self-induction (when 

 the directions of circulation were different) was 23°'l. On 

 the other hand, with the copper wires the large self-induction 

 was 45 o, 0, and the small only l o, 0. Thus, although the 

 manner of connection makes far more relative difference in the 

 case of copper than in the case of iron, the absolute difference, 

 which represents four times the mutual induction of the two 

 wires, is nearly the same, viz. 44 o, for copper and 42 o, for 

 iron. There is here no evidence of any distinction in the 

 mutual induction of iron and copper, the slight want of 

 agreement being easily attributable to different degrees of 



