28 



ELECTRO-MAGNETISM. 



' (78.) It appears, by the result of the 

 experiment related in $ 73, that the 

 eleclro-magnetic influence of the con- 

 ductor takes place equally when the 

 current of electricity is diffused over a 

 considerable surface, as when it is con- 

 centrated in a slender wire. The effects 

 will, of course, be weaker in proportion 

 as it is diffused ; but when the whole of 

 these scattered forces can be brought to 

 bear in the production of any effect, the 

 amount will be the same as when they are 

 concentrated in a smaller space. Thus 

 every filament of which the cylinders in 

 these experiments may be supposed to 

 be resolved, conducts its respective por- 

 tion of the electric current, and con- 

 tributes its share in the production of 

 one common effect, namely, the revolu- 

 tion of the cylinder. 



In like manner it has been found that 

 the stream of electricity, which is pass- 

 ing through the voltaic battery itself 

 from its negative to its positive pole, 

 exhibits the same electro-magnetic pro- 

 perties that it does while passing along 

 the wire which completes the circuit by 

 connecting the two poles; for a mag- 

 netic needle placed in the vicinity of the 

 battery, and in circumstances equally 

 favourable to the action of the current, 

 will be affected in the same way as it 

 is by the wire itself. Now as all action 

 implies a corresponding and equal re- 

 action, it is reasonable to infer that, as 

 the battery produces motion in the mag- 

 net, so the magnet might be made to 

 move the battery, if a sufficiently delicate 

 suspension could be contrived for the 

 latter, so as to render its motion sensi- 

 ble. This could scarcely be effected 

 with a compound battery of any size : 

 but by reducing it to a single plate, 

 making it as light as possible, and sup- 

 porting it on a single point, in the way 

 in which the cylinder was sustained in 

 the last experiment, this object has been 

 accomplished by Ampere. 



(79.) The apparatus he employed for 

 this purpose is represented in section, in 

 fig. 49. It consists of a double cylinder 

 of copper, C C, about two inches and a 

 half in diameter, and the same in height, 

 closed at the bottom, so as to form a 

 vessel capable of holding diluted acid. 

 The whole is supported by an arched 

 plate of metal, which passes across the 

 upper orifice of the inner cylinder on 

 the upper end of a strong magnet, M, 

 which is introduced through the middle 

 of the cylinder. A cylinder of zinc, 

 Z Z, made as light as possible, and sup- 



ported by an arched wire, A, having a 

 steel point proceeding downwards from 

 the middle of its curvature, is introduced 

 between the two plates of the double 

 copper cylinder, so that the steel point 



may rest upon the arched plate of the 

 inner cylinder, and remain balanced in 

 this position. On introducing diluted 

 acid into the copper vessel, a galvanic 

 action immediately commences ; the 

 electric current passing from the zinc to 

 the acid, and ascending from the copper 

 through the pivot back again to the 

 zinc. Hence the zinc is in the situation 

 of a conductor conveying a stream of 

 electricity downwards, and under the 

 influence of the magnetic pole which it 

 surrounds. It will consequently revolve 

 with an accelerated motion, which is at 

 length rendered uniform by the friction 

 of the fluid. 



Mr. Barlow states that he has fre- 

 quently, with this simple apparatus, 

 produced a velocity of one hundred and 

 twenty revolutions in a minute. 



(80.) The theory just explained is pret- 

 tily illustrated by an addition to the pre- 

 ceding apparatus which was made by 

 Mr. J. Marsh ; and which consists in 

 having a second steel point fixed under- 

 neath the upper part of the arch which 

 sustains the copper cylinder : the copper 

 vessel may, by means of this point, be 

 itself balanced on the top of the magnet, 

 while the zinc cylindrical plate is ba- 

 lanced on the former; and each may 

 thus turn round its own centre indepen- 

 dently of the other. This arrangement 



