26 



ELECTRO-MAGNETISM. 



it "will descend into the mercury in the 

 trough, and thence, passing out by the 

 wire below, will proceed through the 

 eup N, and be received by the wire com- 

 municating with the negative end of the 

 battery. The cylinder may therefore be 

 regarded as consisting of a collection of 

 parallel wires, each of which receives 

 from the pole of the magnet placed in 

 the interior an impulse to move in a 

 direction parallel to itself. 



Those on opposite sides of the mag- 

 net will be urged to move in opposite 

 directions; but as their forces act on 

 opposite sides of the axis of motion, they 

 will all concur in their rotatory effect. 

 The whole cylinder is accordingly found 

 to commence revolving as soon as the 



Fig. 46 *. 



electric current is sent through it ; and 

 the resistance it meets with being slight, 

 its velocity soon becomes very consider- 

 able. After what has formerly been 

 said, it is scarcely necessary to add, that 

 the course of its motion is from left to 

 right, or the same with that of the hands 

 of a watch, when the electric current is 

 descending along the cylinder, and when 

 the enclosed part of the magnet is its 

 north pole. 



(74.) The motion is reversed when 

 either of these conditions is reversed. 

 This may be conveniently exemplified 

 in the two poles of the same magnet by 

 employing a horse- shoe magnet, sup- 



See Mr. Walking's Sketch, p. 74.; 



ported vertically in a stand, as shown in 

 fig. 46. Two wooden circular troughs are 

 fixed upon the arms of the magnet, 

 and secured by binding screws. These 

 troughs contain the mercury into which 

 the lower margins of the hollow cylin- 

 ders dip. The upper part of each cylin- 

 der is formed into a hemispherical cup, 

 which is traversed in the middle by a 

 pointed wire, resting below in a small 

 cavity in the centre of the extremity of 

 the magnet contained within the cylin- 

 der, and having at its upper end a small 

 cup to hold mercury. Two other cups, 

 also containing a small quantity of 

 mercury, are supported upon the exter- 

 nal ends of bent wires, which pass 

 through the sides of the circular troughs 

 into the mercury contained in them. 

 Thus a continuous metallic communica- 

 tion is established from one cup to the 

 other, on each side, through each cylin- 

 der which surrounds the different poles 

 of the magnet. If a stream of electri- 

 city from a voltaic battery be made to 

 pass in the same direction in both the 

 cylinders, they will revolve in contrary 

 directions, being acted upon in an op- 

 posite manner by the two poles which 

 they surround. But if the two upper 

 cups be united by a short wire dipping 

 its two ends in the mercury they contain, 

 and the lower, cups be connected, the 

 one \vith the positive, and the other 

 with the negative poles of the battery, 

 the same stream will traverse both sides 

 of the apparatus, passing upwards in 

 one cylinder, and downwards in the 

 other ; and the rotations thence arising 

 will now, from the contrary influences of 

 the two poles, be in the same direction 

 in both the cylinders. 



(75.) The rotation of a conducting 

 body round its own axis, as exhibited 

 in the experiments just related ( 73), 

 throws considerable light upon the cir- 

 cumstances of the experiment before 

 described, in which a magnet was made 

 to rotate about its axis ; for the expla- 

 nation of that experiment will very much 

 depend upon the course which we sup- 

 pose taken by the electrical current 

 during its passage through the magnet. 

 If we supposed it to pass through the 

 interior of the magnet, that is along the 

 axis, and parts adjacent to it, it would 

 occasion rotation by its influence on the 

 parts of the magnet that are situated 

 nearer to the surface, and further from 

 the axis. On the other hand, if we 

 suppose the course of the electric cur- 

 rent to be nearly superficial, then it will 



