a Reetilinear Klectrical Current on itself. 83 
This remarkable experiment has been circumstantially de- 
scribed in more places than one by Ampere himself*; but the 
description is in all essentially alike, and the accounts, being 
nearly contemporaneous, may be assumed to refer to one and 
the same trial, which was made, it appears, at Geneva, and in 
the presence of M. Auguste De la Rive. This last is a circum. 
stance of some importance; for it would perhaps be difficult to 
establish that it has ever been successfully repeated since. The 
authors whom I have consulted usually content themselves with 
the barest citation of Ampére’s authority. The only exception 
which I know of is in Miiller’s Lehrbuch der Physik (vol. u. 
p- 318), where the writer notes the difficulty he found in repeat- 
ing the experiment: ‘“ Ferner muss noch angefiihrt werden dass 
dieser Versuch keineswegs zu den leicht gelingenden gerechnet 
werden kann.” From which I infer that the writer had not suc- 
ceeded in repeating it. He also raises doubts (which seem to be 
reasonable) as to whether, supposing it successful, the conclusion 
of Ampére could with certainty be legitimately drawn from it. 
It appeared to me to be a matter of some interest in connexion 
with the experiment of electrical vibration, to repeat the obser- 
vation of Ampére. I had an apparatus made for the purpose, 
but circumstances have hitherto prevented me from using it; 
and I have requested my successor in the Chair of Natural 
Philosophy, Professor Tait, to make trial of it when he happens 
to have a powerful voltaic battery in action. I did, however, 
attempt the experiment in a different form, and, as it appears to 
me, in one both more sensitive in its indications, and less ambi- 
guous in its interpretationy. 
I fitted up a Coulomb’s torsion apparatus (one that had been 
used for experiments on diamagnetism) in the following way. It 
is represented in outline in fig. 6. A moderately fine platinum 
wire A, about 18 inches long, was used to suspend a wooden 
torsion-rod BC, near one end of which (laid on a piece of paste- 
board fastened to the rod) was poised a copper wire bent in the 
form of a horseshoe, as shown in the figure at ab{. Two strong 
copper wires P,N, the terminals of a Grove’s battery of four 
moderate-sized pairs in good action, were brought into the 
position indicated in the figure, so as to be exactly opposite to, 
and in continuation of, the limbs of the horseshoe. The termi- 
nals P, N were kept firmly in their place, and the extremities a, d, 
* Recueil d’ Observations Electro-dynamiques, 8vo, pp. 285, 318. Also 
Bibliotheque Universelle, vol. xxi. p.47; and Ampére’s Théorie, p. 39 
(this last citedby Dr. Roget in his ‘ Electro-Magnetism,’ art. 187). 
app hus following experiments were made in December 1858 and January 
{ The distance a6 might be half an inch, and the distance from the 
suspension wire was relatively sear than the figure indicates, 
