- 
Revolution of a Magnet without Visible Support. 253 
for the mercurial, the resistance is brought much nearer to the cen- 
tre of motion, and amounts perhaps to very little more than under 
the old plan. In order to obtain a powerful magnet for this pur- 
pose, I make an arrangement similar to that shown in the accom- 
panying figure, which repre- 3 
sents a vertical section through 
the centre of the apparatus. 
a, a, are two helices of insu- 
lated wire, secured to the posts 
EMI 
b, b, is about ten inches long 
and one inch diameter, and =: 
delicately supported upon steel "a= 
pivots. The helices through 7 
which the bar passes, have a SS 
central opening sufficient to sane == == feo 
allow a space of about one- 
sixteenth of an inch between 
them and the surface of the 
bar. Near the centre of the 
thin ferule of silver with = 
which the conductor ¢, also : 
of silver, is in contact by slight pressure. ‘This conductor is in 
connexion with the binding screw cup P. The upper helix has 
one of its ends connected with the bearing of the upper pivot, its 
other with the upper end of the lower helix; and the lower end of 
the latter is connected with the cup N. The course of the current 
1s seen by the arrows, and the whole appears very simple and 
easy of construction, The bar ‘of soft iron and other parts of 
the instrument may be much smaller than the dimensions above 
given, but I have preferred for my own use the very large size 
as better for “class illustration.” The helices being connected 
with twelve or more pairs of Grove’s battery, the bar of soft 
iron becomes powerfully magnetic, and by the action of the cur- 
rent passing through its upper half, revolves with astonishing 
rapidity. By substituting for the current through the upper half 
of the magnet, one of much greater quantity from an independent 
single pair of plates, the effect is greatly increased and exhibits the 
Most rapid rotation I have ever witnessed. But the most interest- 
ing feature in connection with this instrument, is, that during the 
rapid motion the bar is without visible support, the upper and 
lower bearings serving only as guides to keep it in place. By 
imspection of the figure it will be seen that the magnetic bar 
projects farther below the lower helix, than it does above the 
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