ELECTRO-MAGNETISM. 



21 



conducted by a vertical wire of sufficient 

 thickness, which is fixed so as just to 

 dip into the small cistern attached to the 

 magnet. Having reached this point, the 

 current is then diverted from its course 

 by the wire which dips 'into the large 

 cistern, and is thence carried away by the 

 wire which terminates in the cup last 

 described, to such a distance, and in such 

 a direction, as to prevent its acting on 

 the lower pole of the magnet. The 

 magnet will in this manner 'be made to 

 revolve with great rapidity. It is scarcely 

 necessary to remark, that the direction 

 of the rotation will depend both on the 

 direction of the current and on the na- 

 ture of the pole which is acted upon ; so 

 that reversing either of these conditions 

 will occasion a change in the direction 

 of the rotation. Mr. Watkins describes 

 an apparatus by which these opposite 

 rotations may be exhibited in two mag- 

 nets at the same time, and by the same 

 current, by placing the poles of the one 

 in a contrary position to those of the 

 other*. But it is unnecessary to dwell 

 upon these obvious combinations of the 

 more simple forms of the experiment. 



(62.) In the preceding examples, the 

 wire was fixed, and the magnet at liberty 

 to move. But in order to exhibit the 

 revolution of the conducting wire round 

 one of the poles of a magnet, this ar- 

 rangement must be reversed, that is, 

 1he wire must have freedom of motion, 

 and the magnet must be fixed. This 



* A popular sketch of Electro-magnetism and 

 Electro-dynamic.-*. By Francis WatKins. lsL'8. 



Mr. Faraday accomplished by employing 

 the apparatus represented in fig. 36. 

 The glass cup holding the mercury is 

 shallow, and has a tubular stem ; but 

 instead of being filled with a plug, as 

 was the aperture in the former vessel, a 

 small copper socket is placed in it, and 

 retained there by being fastened by a 

 circular plate below, which is cemented 

 to the glass foot, so that no mercury can 

 pass out by it. This plate is tinned and 

 amalgamated on its lower surface, and 

 stands on another plate and wire, just as 

 in the former apparatus. A small cy- 

 lindrical magnet is placed in the socket, 

 at any convenient height, and then mer- 

 cury poured in until it rises so high that 

 nothing but the projecting pole of the 

 magnet is left above its surface at the 

 centre. The forms and relative positions 

 of these parts are seen in the section 

 Jig. 37. The wire which dips into the 

 mercury, and has its lower end amalga- 



Fig. 36. 



Fig. 37. 



mated, may be suspended to a fixed wire, 

 either by a ball and socket joint, con- 

 structed so as to ensure a continuity of 

 metallic conductors, or more simply by 

 means of loops. The best mode of ob- 

 taining a perfect contact, is to make the 

 fixed wire terminate in a small cup con- 

 taining mercury, with its mouth upwards, 

 and to bend the moveable wire into the 

 form of a hook, of which the extremity 

 must be sharpened, and must rest in 

 the mercury on the bottom of the cup, 

 as shown in fig. 38. This latter wire, 

 having full liberty to move, revolves 

 round the pole of the magnet which is 

 above the surface of the mercury, with 

 an accelerated velocity, which afterwards 



