ON PARAMAGNETIC AND DIAMAGNETIC FORCES. 375 



repulsion of certain bodies by the poles of a magnet, 

 is to be ranged as a polar force, beside that of magne- 

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

 When a cylinder of soft iron is placed within a wire 

 helix, and surrounded by an electric current, the anti- 

 thesis of its two ends, or, in other words, its polar ex- 

 citation, 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, un- 

 mixed with the action of the body used to excite the 

 diamagnetic. 



How this has been effected may be illustrated in the 

 following manner : When through an upright helix 

 of covered copper wire, a voltaic current is sent, the top 

 of the helix attracts, while its bottom repels, the same 

 pole of a magnetic needle ; its central point, on the con- 

 trary, is neutral, and exhibits neither attraction nor 

 repulsion. Such a helix is caused to stand between the 



NL 



FIG. 12. 



two poles N'S' of an astatic system. 1 The two magnets 

 SN' and S'N are united by a rigid cross piece at 



1 The reversal of the poles of the two magnets, which were of 

 the same strength, completely annulled the action of the earth as a 

 magnet. 



