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5. Note on a Modification of the Apparatus employed for one 
of Ampere's Fundamental Experiments in Electrodynamics. 
By Professor Tait. 
My attention was recalled by Principal Forbes’s note (read to the 
Royal Society on January 7th), to his request that I should at 
leisure try to repeat Ampere’s experiment for the mutual repulsion 
of two parts of the same straight conductor, by means of an apparatus 
which he had procured for the Natural Philosophy Collection in the 
University. Some days later I tried the experiment, but found that, 
on account of the narrowness of the troughs of mercury, it was im- 
possible to prevent the capillary forces from driving the floating wire 
to the sides of the vessel. I therefore constructed an apparatus in 
which the troughs were two inches wide, the arms of the float being 
also at that distance apart. Making the experiment according to 
Ampere’s method with this arrangement, I found one small Grove’s 
cell sufficient to produce a steady motion of the float from the poles 
of the pile ; in fact, the only difficulty in repeating the experiment 
lies in obtaining a perfectly clean mercurial surface. 
Two objections have been raised against Ampere’s interpretation 
of this experiment, one of which is intimately connected with the 
subject of Principal Forbes’s note. This is the difficulty of ascer- 
taining exactly what takes place where a voltaic current passes from 
one conducting body to another of different material. It is known 
that thermal and thermo-electric effects generally accompany such 
a passage. To get rid of this source of uncertainty, I have repeated 
Ampere’s experiment in a form which excludes it entirely. In this 
form of the experiment the polar conductors and the float form one 
continuous metallic mass with the mercury in the troughs ; the float 
being formed of glass tube filled with mercury, with its extremities 
slightly curved downwards so as nearly to dip under the surface of the 
fluid ; and the wires from the battery being plunged into the upturned 
outward extremities of two glass tubes, which are pushed through 
the ends of the troughs so as to project an inch or two inwards under 
the surface of the mercury. A little practice is requisite to success 
in filling the float and immersing it in the troughs without admitting 
a bubble of air. This float, being heavier than the ordinary copper 
wdre, plunges deeper in the fluid, and encounters more resistance to 
its motion, but, with two small Grove’s cells only, Ampere’s result 
