20 



ELECTRO-MAGNETISM. 



round plate of copper, forming part of 

 the foot of the vessel. A similar plate of 

 copper was fixed to the turned wooden 

 base on which the cup was placed, and 



Fig. S3. 



Fig. 34. 



another piece of strong copper wire, at- 

 tached to it beneath, after proceeding 

 downwards a little way, was made to 

 turn horizontally. The surfaces of these 

 two plates, intended to come together, 

 were tinned and amalgamated, that 

 they might remain longer clean and 

 bright, and afford better contact. The 

 magnet used was of a cylindrical shape, 

 and very powerful, and had its lower 

 pole fastened by a piece of thread to the 

 copper pin at the bottom of the cup. 

 The height of the magnet and length of 

 the thread were so adjusted, that when 

 the cup was nearly filled with clean 

 mercury, the free pole floated almost 

 upright on its surface. The upright 

 wire, communicating with one of the 

 poles of the voltaic battery, and con- 

 ducting the electrical current intended 

 to act on the upper pole of the magnet, 

 passed downwards from the upper 

 branch of a stand, so as to descend to 

 a small depth below the surface of the 

 mercury. Its lower end was amalga- 

 mated, in order to ensure perfect con- 

 tact ; the circuit was completed by 

 making a communication between the 

 lower wire and the other pole of the 

 battery. As soon as the current is thus 

 established through the apparatus, the 

 upper pole of the magnet immediately 

 revolves round the wire which dips into 

 the mercury. As the force which impels 

 it continues to act without diminution, 

 notwithstanding the motion of the mag- 

 net, it operates as an accelerating force ; 

 but the motion of the magnet in a circle 

 giving rise to a centrifugal force, the 

 magnet is carried to a greater distance 

 from the wire, until its increased mo- 

 mentum is compensated by the increased 



resistance of the mercury, at which pe- 

 riod the velocity becomes uniform. 



(60.) The direction of the motion de- 

 pends on the direction of the current, 

 and on the denomination of the pole 

 that is moved by it. If the current 

 descends, the north pole of a magnet 

 revolves from left to right ; that is, in 

 the direction of the hands of a watch. 

 If the revolving pole be the south pole, 

 it moves in the contrary direction. All 

 this is in perfect conformity with what 

 has already been explained in $21, 22, 

 and 23, and illustrated by figs. 7 and 8. 



(61.) With a view of diminishing the 

 resistance to the revolution of the mag- 

 net, which must necessarily take place 

 when it has to revolve in mercury, at- 

 tempts have been made to devise a me- 

 thod of suspending the magnet on a 

 pivot; but the difficulty has always 

 been to provide a proper channel for 

 carrying off the current after it has 

 acted upon one pole of the magnet. 

 It became evident that no solid con- 

 ductor would answer the purpose, as it 

 would always be in the way of the mag- 

 net during its revolution. This object 

 may, however, be accomplished by em- 

 ploying a magnet of the peculiar shape 

 represented in fig. 35, having a double 

 bend in the middle, so that this part is 

 horizontal while the two extremities are 

 kept in a vertical position. The magnet, 

 so shaped, is furnished with an agate 

 cap fixed to the lower side of the middle 

 horizontal portion, resting on a fine 

 point of an upright wire, which is fixed 

 to the base of the apparatus, and upon 

 which the magnet is balanced, so as to 

 allow of its turning freely round. In 

 order to steady its motion, however, a 

 wire loop is attached to the magnet lower 

 down, which embraces the upright wire, 

 and retains that part of the magnet in a 

 position nearly vertical. A small cis- 

 tern, holding mercury, is also fixed upon 

 the magnet at the middle of its upper 

 side, just above the point of suspension. 

 A bent wire, pointed and amalgamated 

 at the end, passes out from this cistern, 

 and dips into a circular trough of mer- 

 cury, which is open in the centre, to 

 allow the magnet to pass freely through 

 the opening, and which is supported on 

 a stage, sustained by means of legs con- 

 necting it with the base. A wire, pro- 

 ceeding from the interior of this circular 

 cistern, passes out of it, and terminates 

 in a cup with mercury. The electrical 

 current, intended to act exclusively upon 

 the upper half of the magnet, is to be 



