Electromagnetic Induction. 339 



points were found as in previous cases. Without again shifting 

 the coil B, a negative curve, the induction-coefficient of which 

 had the same relative value as that of the positive curve, 

 could either be continued or completely traced out by inter- 

 changing the leading wires of the coil D and passing it to the 

 negative region. Thus the curves, whether positive or negative, 

 could either be continued or completely traced out with the 

 thin coils just as with the thick ones ; and a constant value 

 could be given to the mutual induction between the coils C 

 and D in each pair of balancing positions, proportional to, 

 although not the same as the value assigned to M for a given 

 curve. Hence the path traced out by the centre of the coil 

 D was practically the same as that which would have been 

 followed by the centre of the coil D, had it been possible to 

 use it in tracing the same curve in that part of the field. The 

 value of the induction-coefficient of each of the two curves 

 nearest to the origin is uncertain, as the value of z which 

 corresponds to that of x for either of these curves is beyond 

 the range of the curve, fig. 1. A convenient, although un- 

 certain value was given to the induction-coefficient of either 

 of these curves by placing D at the point P and moving C 

 towards until the distance between the centres of C and D 

 was equal to that between the origin and the point where the 

 curve was required to cross the axis of x, then balancing by 

 the coil B and finding the remaining points of the curve. In 

 completing several of these curves for which the value of M 

 was comparatively small, it was found that the mutual in- 

 duction between A and B was greater than that between C 

 and D , even when B was placed at the extreme end of the 

 scale E F. Hence, in order to reduce it in such cases, B 

 was laid down flat on the guide and displaced towards E or 

 F until a balance was obtained ; and although the value of 

 the induction-coefficient was uncertain, the mutual induction 

 between A and B was equal to that between C and D when 

 they occupied a balancing pair of positions previously occupied 

 by C and D, and remained constant while A and B retained 

 the same relative positions. 



The coordinates of the first set of curves of constant induc- 

 tion are given in Table III., and are numbered in accordance 

 with Table II. 



The results obtained by means of the thin coils C and D 

 are distinguished in the Table by being enclosed in square 

 brackets [ ]. 



