Resistance of Compound Conductors. 481 



The more obvious arrangement is one in which both the in- 

 duction and resistance of the branch containing the subject 

 under examination are in every case brought up to the given 

 totals necessary for a balance. To carry this out conveniently 

 we require to be able to add self-induction without altering 

 resistance, and resistance without altering self-induction, and 

 both in a measurable degree. The first demand is easily met. 

 If we include in the circuit the two coils of an induction-com- 

 pensator, connected in series, the self-induction of the whole 

 can be varied in a known manner by rotating the smaller 

 coil. For the self-induction of the instrument, used in this 

 manner, may be regarded as made up of the constant self- 

 inductions of the component coils taken separately, and of 

 twice the positive or negative mutual induction between them. 

 The first part, in consequence of its constancy, need not be 

 regarded ; and thus every degree (within the admissible 

 range) may be taken as representing 2 x 776*3, or 1552*6 

 cm., of self-induction. 



The introduction, or removal, of resistance without altera- 

 tion of self-induction cannot well be carried out with rigour. 

 But in most cases the object can be sufficiently attained with 

 the aid of a resistance-slide of thin German- silver wire. It 

 may be in the form of a nearly close loop, the parallel out- 

 going and return parts being separated by a thin lath of wood. 

 A spring of stout brass wire making contact with both parts 

 short-circuits a greater or less length of the bight. 



In the Wheatstone's quadrilateral, as arranged for these 

 experiments, the adjacent sides R, S are made of similar 

 wires of German silver of equal resistance (^ ohm). 

 Being doubled they give rise to little induction, but the accu- 

 racy of the method is independent of this circumstance. The 

 side P includes the conductor, or combination of conductors, 

 under examination, an induction-compensator, and the resis- 

 tance-slide. The other side, Q, must possess resistance and 

 self-induction greater than any of the conductors to be com- 

 pared, but need not be susceptible of ready and measurable 

 variations. But, as a matter of fact, the second induction- 

 compensator was used in this branch, and gave certain advan- 

 tages in respect of convenience. Sometimes also a rheostat 

 was included ; but during a set of comparisons the condition 

 of this branch was usually maintained constant, the necessary 

 variations being made in P. In order to avoid mutual in- 

 duction between the branches, P and Q were placed at some 

 distance away, being connected with the rest of the apparatus 

 by leads of doubled wire. 



It will be evident that when the interrupter acts in the 



