248 Mr. J. Croll’s Remarks on Ampére’s Eaperiment on 
due to the action of angular currents, and not to a repulsion ex- 
isting between the current in the mercury and the current in the 
branch of the wire in the same straight line. 
Let abcd be the wire floating 
on the mercury, P the point where 
the current enters the mercury, and 
N the point where it leaves it, after 
passing through the wire in the di- 
rection indicated by the arrows. 
The common explanation is, that 
the movement of the wire is due to 
there being in each of the branches 
ab and ed, separately, a repulsion between the current which tra- 
verses them, and the current that is transmitted into the mercury 
before penetrating into the wire or after going out from it; and 
as the current of the mercury and that of the wire are only the 
prolongation of cach other im a right Ime, this 1s considered 
sufficient proof that the one part of the rectilinear current repels 
the other part. 
The following is, however, I think, the true explanation. The 
current Pa in the mercury ts at right angics to the current bc 
in the cross part of the wire. The former current is directed 
towards, and the latter current from the summit of the angle 
abe formed by them. Now, according to Ampére’s well-known 
law of angular currents, the two currents will repel each other. 
In this case the current bc being the moveable one, it will of 
course recede, maintaining a position parallel to itself. This 
eross current be is also at right angles to the current Nd 
on the other side of the glass partition; the former moving 
towards, and the latter from the summit of the angle dcd 
formed by them. These two currents for the same reason will 
repel cach other. The combined influence of the currents P a 
and N d in the mercury will be to cause the cross section of the 
wire which is at right angles to them to recede, maintaining a 
position parallel to itself. It follows, therefore, according to the 
law of angular currents, that Ampére’s experiment must be suc- 
cessful; but then its success does not prove that the parts of a 
rectilinear electrical current are repulsive of each other, but 
simply that a moveable current at right angles to a fixed one is 
repelled when the one current is directed towards, and the other 
from the summit of the angle formed by these two currents. 
It would appear from Dr. Forbes’s experiments, that the dif- 
ferent sections of a rectilinear current are mutually attractive ; 
not repulsive, as is generally supposed. To remoye as much as 
possible all resistance to the motion of the wire, and also to sim- 
plify the conditions of the experiment, imstead of floating the 
