1856.] in Paramagnetic and Diamagnetic Bodies. 161 



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 with the action 

 of the body used to excite the diamagnetic. 



How this may be effected, was illustrated in the following man- 

 ner : — 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, was neutral, and ^^ ^ 



exhibited neither attraction nor ■• '^ 



3S 



repulsion. This helix was caused 

 to stand between the two poles 



N' S' of an astatic magnet ; the 



two magnets S N' and S'N were N ' • 

 united by a rigid cross piece at 

 their centres, and suspended from the point a, so that both magnets 

 swung in the same horizontal plane. It was so arranged that the 

 poles !N'S' were 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 current. Here then we had an excited helix which 

 itself had no action upon the magnets, and we were thus at liberty to 

 examine the action of a body placed within the helix and excited by it, 

 undisturbed by the influence of the latter. The helix was 1 2 inches 

 high, and a cylinder of soft iron 6 inches long suspended from a 

 string and passing over a pulley could be raised or lowered within 

 the helix. When it was so far sunk that its lower end rested 

 upon the table, the upper end found itself between the poles N S 

 attracting one of them, and repelling the other, and consequently 

 deflecting the astatic system in a certain direction. When the cylinder 

 was raised so that the upper end was at the level of the top of the 

 helix, its lower end was between the poles N' S' ; and a deflection op- 

 posed in direction to the former one was the immediate consequence. 

 To render these deflections more visible to the audience, a mirror m, 

 was attached to the system of magnets ; a beam of light thrown 

 upon the mirror was reflected and projected as a bright disk against 

 the wall of the theatre ; the distance of this image from the mirror 

 being considerable, and its angular motion double that of the latter, 

 a very slight motion of the magnet was sufficient to produce a dis- 

 placement of the image through several yards. This then is the 

 principle of the beautiful apparatus* by which the investigation now 



♦ Devised by Prof. W. Weber, and constructed by M. Leyser, of Leipzig. 



M 2 



