18 



ELECTRO-MAGNETISM. 



(50.) Hence it follows that the wire is, 

 in all situations, impelled to move in the 

 direction of the tangent of a circle having 

 its centre in the prolongation of the axis 

 of the magnet, and of which the radius is a 

 mean proportional between the distances 

 of its centre from the two poles. Thus 

 the wire at W, fig. 30, is impelled by the 

 Fig. 30. 



action of the two poles N and S, in the 

 direction of the tangent of the circle 

 of which the centre is at C, in the 

 line NS prolonged, and of which the 

 radius WC is a mean proportional be- 

 tween CS and CN. It will, therefore, 

 revolve in that circle, which will stand in 

 the same relation to the magnetic poles 

 N and S, with regard to the law of elec- 

 tro-magnetic action, that the magnetic 

 curves (See Magnetism, 81) do with 

 regard to the law of magnetic action. 



CHAPTER V. 



Application of the principles to the ex- 

 planation of particular facts. 



(51.) The principles we have derived 

 from the preceding investigation are the 

 foundations of the whole science of 

 electro- magnetism, and furnish the key 

 to the explanation of a vast variety of 

 facts, some of which might appear, with- 

 out an accurate attention to the circum- 

 stances of the case, exceedingly anoma- 

 lous and perplexing. It is evident that 

 they completely accord with the results 

 obtained in the original experiments of 

 Professor Oersted, which could not for 

 some time be clearly understood. 



(52.) In these experiments it will be 

 recollected the wire was horizontal, and 

 applied either above or below the needle, 

 and in a direction parallel to. it. In this 

 case the action of the wire is exerted in 

 the tangent to the circumference of a 

 vertical circle, having the wire for its 

 centre ; and this action being in oppo- 

 site directions upon the two poles, con- 

 spire to give the needle a motion round 

 its axis. But the needle, having already 



a tendency to place itself in the plane of 

 the magnetic meridian, in consequence 

 of the influence of the earth, will arrange 

 itself in a position intermediate between 

 this plane and the position to which it 

 tends by the action of the electric cur- 

 rent. The greater the intensity of the 

 latter force, the greater will be the devia- 

 tion of the needle from the magnetic 

 meridian ; and both the amount and 

 the direction of the deviation will be 

 found on an attentive examination of 

 the results of Oersted's experiments, as 

 already detailed, to be exactly conform- 

 able to theory. 



(53.) When the wire, still kept in a 

 horizontal position, was placed by Oer- 

 sted at right angles to the needle, and 

 over its centre, no visible effect took 

 place, because the actions of the wire 

 upon the two poles were then exactly 

 balanced. But whenever it was brought 

 nearer to one of the poles than to the 

 other, the vertical action being more 

 strongly exerted upon that pole, occa- 

 sioned its elevation or depression, ac- 

 cording to the direction of that action, 

 precisely in the manner which the theory 

 would lead us to expect. 



(54.) Mr. Barlow undertook a series 

 of experiments to determine the devia- 

 tions of a magnetic needle from its na- 

 tural position, produced by a vertical 

 conducting wire under different circum- 

 stances, and deduced from the theory 

 various formulae, by which its amount 

 may be calculated. For the details of 

 his researches, the reader is referred to 

 Mr. Barlow's Essay on Magnetic Attrac- 

 tions *. 



(55.) Of the speculations and hypo- 

 theses to which these extraordinary facts 

 gave rise we shall defer the consideration 

 to a future place, and, confining our 

 attention to the facts themselves, we 

 should here notice the observations of 

 Mr. Faraday, which led to the more 

 striking illustrations of the theory of 

 tangential action we are about to de- 

 scribe. Mr. Faraday states t that on 

 placing the wire perpendicularly, and 

 bringing the needle towards it, in order 

 to ascertain its positions of attraction 

 and repulsion with regard to the wire, 

 instead of finding these to be four, one 

 attractive and one repulsive for each 

 pole, he found them to be eight; that is 

 two attractive and two repulsive for each 

 pole. Thus, allowing the needle to take 



* Second edition, p. 240. 

 f Quarterly Journal of Science, vol. xii. p. 75. 



