82 Prof. J. D. Forbes on the Repulsion of 
phenomena founded on the familiar laws of conduction and ex- 
pansion alone. Dr. Tyndall thinks that the effects of electri- 
city are also to be ascribed to the indirect action of the heat which 
it produces, in the way of expansion. I lave never been satisfied 
with either of these explanations. The conviction to which I 
still hold is that the alleged expansion and contraction cannot 
sensibly operate in the almost infinitely short periods of succes- 
sive contact and separation, and that the effect must be due to 
a molecular impulse of a far more sudden and instantaneous 
character*. We seem to be sufficiently conversant with such im- 
pulses in galvanic, and especially in secondary induced currents. 
This inquiry, however, into the effects of an electrical current 
upon itself, and upon the different portions of one and the same 
conductor conveying it, is one of extraordinary difficulty. We 
cannot fail to be struck by the hesitation with which that great 
Master of this branch of science, Mr. Faraday, expresses himself 
in different parts of his writings respecting them J. 
There is one experiment, quoted m modern treatises on elec- 
tricity, which might seem to throw light on the subject ; and that 
is Ampére’s experiment on the repulsion of one portion of an 
electrical current upon another portion which is a continuation 
of it in the same right line. This repulsion appeared to be the 
necessary complement of Ampére’s theory of the mutual action 
of currents placed in different positions relatively to one another. 
He considered it to be sufficiently demonstrated by the following 
experiment :—A and B (Plate II. fig. 5) are two little troughs 
filled with mercury, or rather a single trough divided lengthwise 
into two by the glass partition CD. Two straight pieces of 
copper wire a,b, united by the bridge c, float on the mercury, 
which, however, they are everywhere prevented from touching 
metallically by means of a coating of sealing-wax, except at the 
extreme ends nearest to A and B, where they are amalgamated, 
and of course touch the mercury. The two poles of a powerful 
battery being imserted in the mercury troughs near the letters 
A, B, the cireuit is evidently completed through the wire acb. 
It is stated that when this is done, the floating wire is repelled 
from the vicinity of the terminal wires of the battery. This 
repulsion is ascribed to the reaction of a rectilinear current from 
A to a through the mercury, upon the continuation of the cur- 
rent in the copper conductor a,.and to a similar action on the 
side 0B. 
* That the electrical vibrations go on during the copious affusion of the 
apparatus with cold water, and that they take place with remarkable energy 
in an almost inexpansible substance like carbon (see my paper cited above), 
are direct arguments in favour of this conclusion. 
tT See particularly the Ninth and Thirteenth Series of his ‘ Researches.’ 
