ELECTRO-MAGNETISM, 



we are acquainted. It does not act in a 

 direction parallel to that of the current 

 which is passing along the wire, nor in 

 any plane passing through that direction. 

 It is evidently exerted in a plane perpen- 

 dicular to the wire, but still it has no 

 tendency to move the poles of the mag- 

 net, in a right or radial line, either 

 directly towards or directly from the 

 wire, as in every other case of attractive 

 or repulsive agency. The peculiarity of 

 its action is that it produces motion in a 

 circular direction all round the wire; 

 that is, in a direction at right angles to 

 the radius, or in the direction of the 

 tangent to a circle described round the 

 wire in a plane perpendicular to it. 

 Hence, as Mr. Barlow has expressed it, 

 the electro-magnetic force exerts a tan- 

 gential action. 



(21.) The direction, in the circumfe- 

 rence of these circles, of the action 

 exerted on any one pole of a magnet by 

 the electrical current which is moving 

 at right angles to the plane of the circles, 



Fig. 7. 



is determined by the direction of the 

 current. If we suppose the conducting 

 wire to be placed in a vertical situation, 

 as shown in fig. 7, p n, and the current 

 of positive electricity to be descending 

 through it, or moving from p to n (the 

 negative electricity moving, of course, 

 in the contrary direction, or ascending), 

 and if through any point C in that wire, 

 the plane NN be taken perpendicular 

 to p n, that is, in the present case, a 

 horizontal plane ; and lastly, if any num- 

 ber of circles be described in that plane 

 having C for their common centre, then 

 the action of the current in the wire 

 upon the north pole of a magnet, si- 

 tuated any where in that plane, will be 

 to move it in the line of the tangent to 

 the circle which passes through it, and 

 in the direction denoted by the arrows 

 in the figure ; that is, from left to right 

 in the remote part of the circle, and from 

 right to left in the nearer part. In other 

 words, the motions impressed will be in 

 a direction corresponding to those of 



Fig. 8. 



the hands of a watch having the dial 

 towards the positive pole of the Voltaic 

 battery. 



(22.) When the direction of the cur- 

 rent is reversed, the wire still preserving 

 its vertical position, the direction of the 

 action is also reversed ; and the circular 

 motions produced correspond to the 

 movements of the hands of a watch with 

 its face downwards ; that is, still looking 

 towards the positive electrical pole. 



(23.) The actions of either" the de- 

 scending or ascending electrical current 

 upon the south pole of a magnet are 

 exactly the reverse of those which are 

 exerted on the north pole. Fig. 8 re- 

 presents the action of the current mov- 



ing from p to n, on the south pole ; 

 which is directed, as may be seen, from 

 right to left in that part of the circle 

 which is opposite to the wire, and which 

 would, therefore, impel the south pole 

 in a direction contrary to that of the 

 hands of a watch. On reversing the 

 direction of the current these effects will 

 again be reversed. 



(24.) It is evident that in the course 

 of experiments on electro- magnetism, 

 the current and^ magnetic poles. may be 

 presented to our observation in a great 

 variety of relative positions ; and it will 

 be found not very easy to retain a per- 

 fect recollection of the way in which the 

 force should act conformably to the rule 



