ELECTRO-MAGNETISM. 



becomes uniform, from the increasing re- 

 sistance of the fluid ; the direction of 

 the motion being determined by the 

 principles already laid down in 26, and 

 exemplified by jigs. 9, 10, and 11. 



Fig. 38. 



(63.) Mr. Faraday also contrived a 

 small apparatus, answering a similar 

 purpose with the last, and in which the 

 wire revolves very rapidly, with a very 

 small voltaic power. It consists of a 

 piece of glass tube, GG, fig. 39, the lower 

 end of which is closed by a cork, 

 Fie:. 39. through which a small 

 piece of soft iron wire is 

 passed, so as to project 

 above and below the cork. 

 A little mercury is then 

 poured in, to form a chan- 

 nel between the iron wire 

 and the glass tube. The 

 upper orifice is also closed 

 by a cork, through which 

 a piece of platinum wire 

 passes, and terminates be- 

 low by a loop ; another 

 piece of wire hangs from 

 this by a loop, and its 

 lower end, which dips a 

 very little way into the 

 mercury, being amalga- 

 mated, it is preserved from 

 adhering either to the iron 

 wire or to the glass. When 

 even a feeble voltaic com- 

 bination is connected with 

 the upper and lower ends of 

 this apparatus,and the pole 

 of a magnet is placed in 

 contact with the external 

 M end of the iron wire M, 



the moveable wire within rapidly ro- 

 tates round the temporary magnet thus 

 formed by induction at the moment, and 

 by changing either the connexion or the 

 pole of the magnet in contact with the 

 iron, the direction of the motion itself is 

 changed. This apparatus has been 

 made so small as to produce rapid revo- 



lutions, by the action of two plates of 

 copper and zinc, containing not more 

 than a square inch of surface each. 



(64.) A still more simple mode of ex- 

 hibiting the rotation of the wire, is to 

 employ, instead of a pierced cup, a wide 

 and very shallow vessel, as a tea-saucer, 

 for containing the mercury, and to bring 

 a strong magnet underneath as near to 

 it as possible. It may even be placed 

 under the table on which the vessel is 

 laid. Under these circumstances, the 

 revolution of a wire, allowed to dip into 

 the mercury as before, will take place as 

 soon as it is placed in the voltaic circuit. 

 The effect is the same, whether the 

 magnet be held in a horizontal or vertical 

 position, or inclined at any angle, pro- 

 vided the magnet be of sufficient length, 

 so that the influence of the other pole 

 may not act sensibly upon the wire. 



(65.) An apparatus was constructed 

 by Mr. Griffiths, for exhibiting, in like 

 manner, the simultaneous revolution of 

 two conducting wires round the opposite 

 poles of magnets. Two copper wires, 

 suspended so as to move freely, .were 

 made to dip into a shallow vessel con- 

 taining mercury, in which were fixed two 

 bar magnets, with their opposite poles 

 raised above the surface. On making 

 the connexion between the battery and 

 the apparatus, the wires revolved round 

 the magnets simultaneously, but in op- 

 posite directions*. 



(66.) The two forms of electro-mag- 

 netic rotation which have now been 

 described, were exhibited at the same 



* An apparatus of this kind was exhibited by 

 Mr. Barlow, at the London Institution, in 1823, in 

 a course of lectures which he there gave on Elec- 

 tro-magnetism. 



