On the Conjugate Positions of hvo Circular Coils of Wire. 413 



With the first arrangement of apparatus it was possible to 

 place the coils so as to get complete silence in the telephone. 

 With the powerful current from twelve Grove's cells and the 

 key for making and breaking the circuity however, the silence 

 is not absolute ; but in the positions which give a minimum 

 of sound the sound is very faint, being just audible and no 

 more. This faint sound may perhaps be accounted for partly 

 because the different convolutions of wire in the secondary 

 coil experience slightly different inductive effects from the pri- 

 mary one, and partly because it is difficult to adjust the posi- 

 tions of the coils with any great degree of accuracy without 

 having special arrangements for the purpose. 



Now it w T as found that the various conjugate positions in 

 which the secondary coil could be placed in the neighbourhood 

 of the primary one were situated in a path along which it 

 could be moved either towards or away from the primary 

 coil without sounds being heard in the telephone, but that 

 with a slight deviation from this path to either side the sounds 

 were again heard. 



In order to ascertain whether the direction of the currents 

 in the secondary circuit was reversed when the coil was 

 moved from one side of the path to the other, a delicate re- 

 flecting galvanometer was substituted for the telephone, and 

 the position of the coil so adjusted that on making and break- 

 ing the circuit no deflection of the galvanometer was observed. 

 The coil was now moved slightly away from this position, say, 

 towards the right ; and the direction of the deflection of the 

 galvanometer on making contact was noted, that on breaking 

 being, of course, in the opposite direction. The coil was now 

 moved towards the left to the other side of the path, and 

 the direction of the deflections again observed ; and it was 

 found that they were now reversed. We may therefore infer 

 that this path (which, if it could be fully traced, would of 

 course constitute a surface of revolution about the axis of the 

 primary coil) divides space into two regions, in one of which 

 the inductive action of the primary coil has the opposite di- 

 rection to what it has in the other. 



This path appeared to be slightly curved ; and it seemed as 

 if a part of it might very readily be traced. The part which 

 appeared to be best suited for this purpose was that along 

 which the secondary coil has to pass while being moved away 

 from contact with the primary one parallel to it to a position 

 at some distance from it, as here the inductive effect is 

 greatest, and therefore any deviation of the coil from the 

 proper position in the path is most easily detected. As the 

 coils are further separated, however, the position of the path 

 becomes more difficult to trace, until at last we lose it 

 altogether. 



