246 FRAGMENTS OF SCIENCE. 



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 wire helix, and sur- 

 rounded 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 sub- 

 stituted for the cylinder of iron, and its state similarly ex- 

 amined. 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. 



Ho\v 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 







FIG. 10. 



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 the two poles if' s' 

 of an astatic system.* The two magnets s N' and s' sr are 

 united by a rigid cross piece at theii r centers, and are sus- 

 pended from the point a, so that both magnets swing in 

 the same horizontal plane. It is so arranged that the pok'.s 

 N' s' are opposite to the central or neutral point of the 

 helix, so that when a current is sent through the latter, the 

 magnets, as before explained, are unaffected. Here, then, 

 we have an excited helix which itself has no action upon 

 the magnets, and we are thus enabled to examine the 

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



* 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. 



