of two Circular Cods of Wire. 415 



will be found by referring to the lines of force due to a cir- 

 cular current. These lines are represented by closed curves 

 surrounding the section of the wire through which the current 

 flows : and they are given in Prof. Clerk Maxwell's work 6 On 

 Electricity and Magnetism/ vol. ii. pi. 18. If we draw tan- 

 gents to them parallel to the plane of the circular current, it 

 will be found that the points where they touch are situated in 

 a curve somewhat similar to that which we have found by ex- 

 periment. The two curves, however, will not be found to 

 coincide exactly, except in the case where the secondary coil 

 does not enclose a space — that is to say, when its diameter is 

 infinitely small. With respect to the curve drawn through 

 the points of contact of the tangents to the lines of force, it 

 will be seen that the direction of all these lines between the 

 curve and the axis of the circular current is away from, and 

 that their direction on the other side of the curve is towards 

 the plane of the circular current : hence on opposite sides of 

 the curve their tendency is to produce currents in opposite 

 directions. 



If the curve is now supposed to revolve round the axis of 

 the circular current, all lines of force enclosed by the surface 

 generated will tend to produce currents in one direction, 

 while all lines outside the surface will tend to produce currents 

 in the opposite direction. Therefore, when the secondary coil 

 is so situated with respect to this surface that as many lines 

 of force pass through it in one direetion as in the other, the 

 resultant inductive effect on it will be zero ; and this will be 

 the case when it occupies any of the conjugate positions*. 



It is evident from this, therefore, that we may combine the 

 coils in several ways for the suppression of inductive effects : — 

 first, by placing them close together face to face with their 

 axes coincident, and so arranged that one of them may be 

 moved across the face of the other parallel to their planes till 

 a balance is obtained ; secondly, by placing them at some dis- 

 tance apart with their planes parallel and their axes coincident, 

 and so arranged that if their planes are vertical each of them 

 may be made to rotate round its vertical diameter ; then if 

 they are joined together when their axes are coincident, and 

 combined like parallel rulers, they may be made to rotate 

 together until a balance is obtained. With regard to this 



* In what precedes, the planes of the coils have been always supposed 

 to be parallel to each other : but it evidently follows from the reasoning 

 here indicated that, if any set of parallel tangents be drawn to the lines of 

 force and a cur re be traced through the points of contact, an infinitely 

 small coil would experience no inductive effect if it were placed with its 

 centre anywhere in this curve, and with its plane parallel to the given set 

 of tangents. 



