250 Prof. Challis on Theories of Magnetism 
any two sections of the current attract each other when the current 
is passing, is always as the amount of the resistance which opposes 
this attraction. The attraction must always exceed the resist- 
ance, or else there can be no current. What does all this mean 
if we do not admit that the sections of the current mutually 
attract each other ? 
The same difference of electric state exists in the various see- 
tions of the pile itself; for we know that there is always an ex- 
cess of positive electricity at the one pole, and of negative at the 
other, and these electricities must tend to unite through the pile 
itself. But there is opposed to the attractive force of the elec- 
tricities for uniting, not only the resistance in the pile itself, but 
also the electromotive force which decomposes the electricity. 
This electromotive force must always exceed the attraction of the 
electricities: this must be admitted; for unless the foree which 
separates the electricities is greater than the attraetive force 
which tends to unite them, there could be no decomposition of 
the electricity, aud of course no current. 
In the pile itself there are two forces—one tending to unite 
the various sections of the current, and the other tending to 
separate them; the latter force being the strongest, the sections 
of the current in the pile will mutually repeleach other. In the 
external conductor which unites the poles of the pile, the latter 
force has no existence; hence the various sections of the current 
in the conductor are mutually attractive. By virtue of the 
repulsion in the pile and the attraction in the conductor, the 
electricity decomposes in the former and unites in the latter; and 
this constitutes what we call an electric current. 
XXXIX. On Theories of Magnetism and other Forces, in reply to 
Remarks by Professor Maxwell. By J. Cuarus, F.R.S., 
F.R.A.S., Plumian Professor of Astronomy and Experimental 
Philosophy in the University of Cambridge *. 
N an article on “ Molecular Vortices applied to Magnetic 
Phenomena,” contained in the March Number of the Phi- 
losophical Magazine, Professor Maxwell has made, respecting 
certain points of the general physical theory which I have lately 
proposed, some remarks which call fora reply. I refer chiefly 
to two paragraphs in p. 163, the first of which is as follows :— 
“ Currents, issuing from the north pole and entering the south 
pole of a magnet, or circulating round an electric current, have 
the advantage of representing correctly the geometrical arrange- 
ment of the lines of force, if we could account for the pheno- 
* Communicated by the Author. 
