Chemistry and Physics. 



265 



I 





t 



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;^^ 



no motion of the mass towards the pole referred to ; and llie reason was 

 known to be, that although the marked end of the needle was attracted 

 by the north pole, the unmarked end was repelled by an equal quaiitity, 

 and these two equal and opposite forces neutralized each other as regards 

 the production of a motion of translation. When the pole of an ordi- 

 nary magnet was brought to act upon the swimming needle, the latter 

 was attracted, — the reason boing that the atti-acted end of the needle 

 being much nearer to the pole of the magnet than the repelled end, the 

 force of attraction was the more powerful of the two; but in the case of 

 the earth, the pole being so distant, the length of the needle was prac- 

 tically zero. In like manner, when a piece of iron is presented to a mag- 

 net, the nearer parts are attracted, while the more distant parts are re- 

 pelled ; and because the attracted portions are nearer to the magnet than 

 the repelled ones, w^e have a balance in favor of attraction. JTere then 

 IS the most wonderful characteristic of the magnetic force, which distin- 

 guishes it from that of gravitation. The latter is a simple unpolar force, 

 while the former is duplex or polar. Were gravitation like magnetism, a 

 stone would no more fall to the ground than a piece of iron towards the 

 north magnetic pole: and thus, however rich in consequences the suppo- 

 sition of Kepler and others may have been, it was clear that a force like 

 that of magnetism would not be able to transact the business of the 

 universe. 



The object of the evening's discourse was to inquire whether the force 

 of diaraagnetism, wliich manifested itself as a repulsion of certain bodieft 

 by the poles of a magnet, was to be ranged as a polar force, beside that 

 of magnetism ; or as an unpolar force, beside that of gravitation. When 

 a cylinder of soft iron is placed within a hehx, and surrounded by an 

 electric current, the antithesis of its two ends, or in other words, its polar 

 excitation, is at once manifested by its action upon a magnetic needle; 

 and it may be asked why a cylinder of bismuth may not be substituted 

 for the cylinder of iron, and its state similarly examined. The reason is, 

 that the excitement of the bismuth is so feeble, that it would be quite 

 masked by that of the helix in which it is enclosed; and the problem 

 that now meets us is, so to excite a diamagnetic body that the pure action 

 of the body upon a magnetic needle may be observed, unmixed M'ith the 

 action of the body used to excite the diamagnetic. 



How this may be effected, was illustrated in the following manner: — 

 an upright helix of covered copper wire was placed upon the table, and 

 it was shown that the top of the helix attracted, while its bottom repelled, 

 the same pole of a magnetic needle ; its central point, on the contrary, 

 ^as neutral, and exhibited neither attrac- 

 tion nor repulsion. This helix was caused 



S[ 



:x 



a 



raS 



to stand between the two poles W S' of 



an astatic magnet ; the two magnets S N' 



and S'N were united by a rigid cross ^^ ]| ^^^J 



piece at their centres, and suspended from ^'' 



+1 • 7 1 7/i 



tne pomt a, so that both magnets swung 



in the same horizontal plane. It was so arranged that the poles N'S' 

 ^ere opposite to the central or neutral point of the helix, so that when a 

 current was sent through the latter, the magnets were unaffected by the 



SECOND SERIES, VOL. XXIV, NO. 71.— SEPT., 1957, 



34 



