and on the Theory of Magnetism. 77 



placed upside down for an upper centre of motion; for a 

 lower centre, a similar cup was made of copper, into which a 

 little mercury was put; this was placed in ajar of water under 

 the former centre. A piece of copper wire was then bent into the 

 form of a crank, its ends amalgamated, and the distances being 

 arranged, they were placed in the cups. To prevent too much 

 friction from the weight of the wire on the lower cup, it had 

 been passed through a cork duly adjusted in size, and that being 

 pushed down on the wire till immersed in the water, the friction 

 became very little, and the wire very mobile yet with good 

 contacts. The plates being then connected with the two cups, 

 the apparatus was completed. In this state, a magnetic pole 

 being brought to the centre of motion of the crank, the wire 

 immediately made an effort to revolve until it struck the mag* 

 net, and that being rapidly brought round to the other side, the 

 wire again made a revolution, giving evidence that it would 

 have gone round continually but for the extension of the magnet 

 on the outside. To do away with this impediment, the wire 

 and lower metal cup were removed, and a deep basin of mer- 

 cury placed beneath ; at the bottom of this was a piece of wax, 

 and a small round bar magnet was stuck upright in it, so that one 

 pole was about half or three-fourths of an inch above the sur- 

 face of the mercury, and directly under the silver cup. A 

 straight piece of copper wire, long enough to reach from the 

 cup, and dip about half an inch into the mercury, had its ends 

 amalgamated, and a small round piece of cork fixed on to one 

 of them to make it more buoyant ; this being dipped in the 

 mercury close beside the magnet, and the other end 

 placed under the little cup, the wire remained upright, for the 

 adhesion of the cork to the magnet was sufficient for that pur- 

 pose, and yet at its lower end had freedom of motion round the 

 pole. The connection being now made from the plates to the 

 upper cup, and to the mercury below, the wire immediately 

 began to revolve round the pole of the magnet, and continued 

 to do so as long as the connexion was continued. 



When it was wished to give a large diameter to the circle 

 described by the wire, the cork was moved from the magnet. 



