1G 



ELECTRO-MAGNETISM, 



so that it may be balanced horizontally, 

 and bringing it vvilhin a certain distance 

 of a vertical conducting wire P n; if the 

 electrical current be descending in that 



wire the needle will place itself so that 

 the north pole N will be to the right, and 

 the south pole to the left of a spectator 

 conceived to be placed in the situation 

 of the wire and looking towards the 

 needle, as shown in the figure ; whereas 

 if the needle be before the wire as at 

 N'S' the poles will have a reverse po- 

 sition. In both cases the needle will be 

 impelled towards the wire, as shown by 

 the inclination of the thread by which 

 it is suspended. 



' (46.) When, on the other hand, the 

 needle is removed to a considerable dis- 

 tance from the wire, or what comes to 



the same thing, when a very short 

 needle, s n, is taken and carried round 

 the wire P N,Jig. 25, in a circle, its poles 

 will always preserve the same relative 

 situation, as indicated by the letters in 

 the figure, each being turned in the 

 direction in which they are respectively 

 urged to move round the circumference 

 by the tangential force. But the ten- 

 dency to approach the wire will be quite 

 insensible, in consequence of the angle 

 formed by the directions of the two 

 forces being so nearly equal to two right 

 angles. 



(47.) When the wire is placed in any 

 part of the circumference of the circle, 

 having for its centre the centre of the 

 needle, and passing ^through the poles, 

 the resultant of the two forces C s and 

 Cn (Jig. 26), has the exact direction 

 of the line CW; and therefore, neither 



in this, nor in the preceding case, is 

 there any rotatory force in operation. 

 But, in the present case, the force CW 

 being oblique to the axis of the needle 

 SN, a part of that force is exerted in 

 moving the needle in the direction of its 

 length, from C towards S, and in bring- 

 ing the centre C opposite to W ; so that 

 it will not rest until that centre comes 

 in contact with the wire, as shown in 

 fg. 27. A similar tendency in the cen- 

 tre of the needle to move towards the 

 wire takes place in all other situations 



Fig. 27. 



