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. 
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 
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 N" s' 
of an astatic system.* The two magnets s N' arid s' N" are 
united by a rigid cross piece at their 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 poles 
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. 
