ELECTRO-MAGNETISM. 



29 



Fig. 50. 



is represented in the section fig. 50. As 

 the electric current ascends in the 

 copper cylinder, while it descends along 

 the zinc, the former 

 will be urged by 

 the magnet in its 

 interior, to revolve 

 in a direction con- 

 trary to the motion 

 of the zinc cylin- 

 der. The velocity 

 of the copper vessel 

 is, however, much 

 smaller than that 

 of the zinc, not 

 only from its greater 

 weight, and from 

 carrying besides the 

 whole quantity of 

 acid, but also from 

 the friction of its 

 pivot being in- 

 creased by the 

 weight of the zinc 

 plate which that pi- 

 vot has to support. 



^ ^^^^^^^ - _ - ____ In this double re- 

 volution, also, the 



velocity of the zinc plate is further re- 

 tarded by the increased resistance it 

 meets with from the fluid which is 

 moving in a contrary direction. 



(81.) Mr. Watkins has applied an ap- 

 paratus of this kind to each of the poles 

 of a horse-shoe magnet, firmly fixed in 

 a metal stand at its bent part, as shewn 

 in fig. 51. The upper ends of the mag- 

 net are furnished with agate cups for 

 receiving the steel points on which the 

 apparatus is supported. The wire it- 

 self traverses the arch affixed to the 

 copper vessel, and terminates in a point 

 at its upper extremity also, so that the 

 arch connected with the zinc plate rests 

 upon it. When the apparatus is brought 

 into action by charging the vessels with 

 acid, the four cylinders are seen to re- 

 volve on their axes, the two copper 

 vessels turning in opposite directions, and 

 the two zinc cylinders turning in direc- 

 tions opposite to these, and of course 

 also contrary to each other: the rapidity 

 of their revolutions depending on the 

 power of the magnet, on the strength of 

 the diluted acid, and on the delicacy of 

 their suspension. 



(82.) Horse-shoe magnets may also 

 be conveniently employed for combin- 

 ing the, effects of both poles in giving 

 motion to a conducting wire. The 

 operation, of the two poles being in con- 



trary directions at their opposite sides, 

 they will, on the other hand, conspire 

 in producing the same effect upon a 

 wire placed between them. Thus each 



g. 51. 



of the conducting wires p n, p' n', figs. 

 52, 53, in which the electric current is 



Fis. 52. 



Fig. 53. 



descending' from p to s, when placed 

 between the magnetic poles N and S, 

 the former being north, and the latter 



