ELECTRO-MAGNETISM. 



19 



its position of equilibrium across the 

 wire, and then drawing away slowly the 

 support of the needle from the wire, so 

 as to bring: the north pole, for instance, 

 nearer to it, there was attraction ; but 

 on moving it a little farther, so that the 

 end of the needle was the point nearest 

 to the wire, repulsion took place, al- 

 though the wire was still on the same 

 side of the needle. When the wire was 

 on the other side of the same pole of 

 the needle, it repelled it when opposite 

 to most parts between the centre and 

 the end ; but there was a small portion, 

 at the very end, where attraction took 

 place. 



(56.) Fig. 31 exhibits a compendious 

 view of the relative situations of the 

 needle and wire in these experiments ; 



Fig. 31. 



the electric * current being supposed 

 to ( descend along the vertical wire, 

 p n, represented in eight different 

 positions ; the letters A, a, R, r, denot- 

 ing respectively the apparent action 

 (whether attractive or repulsive) ex- 

 erted in each of these positions. A 

 reference to Jig. 22, and the general re- 

 sults stated in 40, will sufficiently ex- 

 plain the facts mentioned by Mr. Fara- 

 day, if we take into account a circum- 

 stance which very generally obtains in 

 needles of the pointed shape of those 

 employed in the experiment; namely, 

 that the centre of the active portion of 

 each half of the needle, or its true pole, 

 is not situated at the very extremity, 

 but at some point near it, and towards 

 the centre of the needle. Thus the wires 

 in the extreme positions at the ends of 

 the needle were in fact placed beyond 

 the poles, and corresponded in their si- 

 tuation to points out of the circle passing 

 through those poles, which is the circle 

 given in/g-. 22. 



(57.) The reaction of the needle on 

 the wire in these situations was also 

 pointed out by Mr. Faraday, and illus- 

 trated by reference to the following 

 figure (32), which represents horizontal 



sections of the wire in different positions 

 with regard to the needle, balanced in 

 its centre C. They are marked A or R, 



Fig. 32. 



As S K 



according as they appear to attract or 

 repel the adjacent poles S and N ; and 

 the arrow-heads indicate the directions 

 of the circular motion which resulted. 



(58.) Mr. Faraday justly concluded 

 from these facts, that there is no real 

 attraction or repulsion between the wire 

 and either pole of a magnet, the actions 

 which imitate these effects being of a 

 compound nature ; and he also inferred 

 that the wire ought to revolve round a 

 magnetic pole, and a magnetic pole 

 round a wire, if proper means could be 

 devised for giving effect to these tenden- 

 cies, and for isolating the operation of 

 a single pole. For the first idea of the 

 possibility of the rotations of an electro- 

 magnetic wire round its axis by the ap- 

 proach of a magnet, we are indebted 

 to the sagacity of Dr. Wollaston*, who 

 did not, however, succeed in producing 

 this effect in the experiments which he 

 made for that purpose. 



CHAPTER VI. 

 Electro-magnetic Rotations. 



(59.) The continued revolution of one 

 of the poles of a magnet round a ver- 

 tical conducting wire was produced by 

 Mr. Faraday in the following manner ( : 

 That the action of the wire might be 

 limited to the pole in question, the whole 

 magnet, with the exception of that ex- 

 tremity in which the pole was situated, 

 was immersed in mercury, its lower end 

 being attached by a thread to the bottom 

 of the vessel which contained the mer- 

 cury, the conducting wire being made 

 to pass down into the mercury, imme- 

 diately above the place where the copper 

 wire was fixed to the vessel. This ap- 

 paratus is represented mjig. 33, and a 

 section of it shown in fig. 34. 



For the purpose of directing the elec- 

 trical current through the mercury, a 

 hole was drilled at the bottom of the 

 cup, into which a copper pin was 

 ground tight, projecting upwards a little 

 way into the cup, and rivetted to a small 



* Philosophical Transactions for 1823, p. 158. 

 t Quarterly Journal of Science, xii. p. 283, 

 C2 



