PARAMAGNETIC AND DIAMAGNETIC FORCES. 333 



repulsion of certain bodies by the poles of a magnet, 

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

 ism; 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 an- 

 tithesis 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, 

 unmixed 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 contrary, is neutral, and exhibits neither attraction 

 nor repulsion. Such a helix is caused to stand between 



Fio. 10. 



the two poles N* s' of an astatic system.* The two 

 magnets s N' and s' N are united by a rigid cross piece 



* 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. 



