8388 Prof. Maxwell on the Theory of Molecular Vortices 
Here the near coincidence of the results in the first and third 
columns shows that the relation between & and T may be ap- 
proximately expressed by the formula 
’ le 1 1B) . . . e y ) 
LI. On Physical Lines of Force. By J.C. Maxwetu, Professor 
of Natural Philosophy in King’s College, London. 
[With a Plate. ] 
Part Il.—The Theory of Molecular Vertices applied to Electric 
Currents. 
[Concluded from p. 291.] 
Ae an example of the action of the vortices in producing in- 
duced currents, let us take the following case:—Let B, 
Pl. V. fig. 3, be a circular rg, of uniform section, lapped uni- 
formly with covered wire. It may be shown that if an electric 
current is passed through this wire, a magnet placed within the 
coil of wire will be strongly affected, but no magnetic effect will 
be produced on any external point. The effect will be that of 
a magnet bent round till its two poles are in contact. 
If the coil is properly made, no effect on a magnet placed out- 
side it can be discovered, whether the current is kept constant or 
made to vary in strength; but if a conducting wire C be made 
to embrace the rmg any number of times, an electromotive force 
will act on that wire whenever the current in the coil is made to 
vary; and if the circuit be closed, there will be an actual current 
in the wire C. 
This experiment shows that, in order to produce the electro- 
motive force, it is not necessary that the conducting wire should 
be placed in a field of magnetic force, or that lines of magnetic 
force should pass through the substance of the wire or near it. 
All that is required is that lines of force should pass through the 
circuit of the conductor, and that these lines of force should vary 
in quantity during the experiment. 
In this case the vortices, of which we suppose the lines of 
magnetic force to consist, are all within the hollow of the rig, 
and outside the ring all is at rest. If there is no conducting 
circuit embracing the ring, then, when the primary current is 
made or broken, there is no action outside the ring, except an in- 
stantaneous pressure between the particles and the vortices which 
they separate. If there is a continuous conducting circuit em- 
bracing the ring, then, when the primary current is made, there 
will be a current*in the opposite direction through C; and when 
