ELECTRO-MAGNETISM. 



info a circular form, as shewn \njig. 61, 



the magnetic pole P, placed in the centre 



Fig.Bl. 



of the circle, or in a line passing through 

 that centre, and at right angles to its 

 plane, would be impelled in one uniform 

 direction by an electric current trans- 

 mitted thro'ugh the wire, in every part 

 of its course along that circular bend. 

 (95.) Supposing'it were possible for 

 the current to move in a perfect circle, 

 its direction being that indicated by the 

 arrows in figures 62 and 63, the north 

 pole of a magnet placed in its centre 

 would move to the right, and the south 



pole to the left ; as shewn by the arrows 

 at N and S. If the north pole of a 

 magnet, therefore, were presented to the 

 right hand side of this circular current, 

 it would tend to move away from it, 

 having the appearance of being repelled : 

 and since a similar and reciprocal action 

 takes place between the magnetic pole 

 and the electric current, the latter, 

 together with the wire which conveys it, 

 will, if at liberty to move, recede from 

 the magnet, or appear to be repelled by 

 it. Just the contrary would happen if 

 the south pole of a magnet were pre- 

 sented on the same side ; that is, there 

 would be the appearance of a mutual 

 attraction between them. But when 

 either of these poles is presented on the 

 other side of the plane of the circular cur- 

 rent, effects of an opposite kind are pro- 

 duced : the north pole appears to attract, 

 and the south pole to repel. If the 

 north pole, which thus appears to attract 

 on one side, be brought nearer and nearer 

 to tho, plane of the circle, the apparent 

 attraction goes on increasing, till it 



reaches that* plane ; but the moment it 

 passes through it and comes on the 

 other side, a repulsion equally strong 

 with the former attraction commences ; 

 gradually diminishing as the distance 

 from the plane increases. 

 v (9G.) This hypothetical case may in 

 some measure be realized in a very in- 

 genious apparatus invented by M. De 

 la Rive *, and which is shown in Jig. 64. 

 It consists of a small galvanic battery, 

 formed by a pair of zinc and copper 

 plates, Z and C, attached to a cork of 



Fig. C4. 



sufficient size to enable the whole ap- 

 paratus to float on the acidulated 

 water which is to act upon the zinc. 

 Each of the metallic plates is about half 

 an inch wide, and extends nearly two 

 inches below the cork, through which 

 its upper end is made to pass. A piece 

 of copper wire, W, covered with silk 

 thread, is affixed to the copper plate, 

 and passing upwards through the cork, 

 is bent into the form of a circle of about 

 an inch in diameter, so that the other 

 end returns into the cork and may be 

 soldered to the plate of zinc. In the 

 galvanic circuit which is thus formed by 

 the acid and the plates of zinc and cop- 

 per connected by the wire, an electric cur- 

 rent is determined from the copperplate, 

 along the circular wire, to the zinc plate, 

 as shown by the arrows ; and the mo- 

 bility of the floating apparatus affords 

 the best opportunity of exhibiting all 

 the effects of the attractive and repul- 

 sive tendencies we have just been de- 

 scribing, when a magnet is brought 

 near it on either side. It is proper to 

 remark that the instrument is rendered 

 more powerful by causing the wire to 



* This apparatus is described in the Bibli- 

 otheque Universe!, vol. xvi. p. L'Ol ; and in the 

 Quarterly Journal of Science, vol. xii. p. J84 



