1898-99.] 
Lord Kelvin on Magnetism. 
631 
Magnetism and Molecular Rotation. 
By Lord Kelvin, P.R.S.E. 
(Read July 17, 1899.) 
§ 1. Consider the induction of an electric current in an endless 
wire when a magnetic field is generated around it, For simplicity, 
let the wire he circular and the diameter of its section very small 
in comparison with that of the ring. The time-integral of the 
electromotive force in the circuit is 2AM, if A denote the area of 
the ring and M the component perpendicular to its plane, of the 
magnetic force coming into existence. This is true whatever he 
the shape of the ring, provided it is all in one plane. Now, 
adopting the idea of two electricities, vitreous and resinous, we 
must imagine an electric current of strength C to consist of 
currents of vitreous and resinous electricities in opposite direc- 
tions, each of strength JC. Hence the time-integrals of the 
opposite electromotive forces on units of the equal vitreous and 
resinous electricities are each equal to AM. 
§ 2. Substitute now for our metal wire an endless tube of 
non-conducting matter, vitreously electrified, and filled with an 
incompressible non-conducting fluid, electrified with an equal 
quantity, e, of resinous electricity. The fluid and the containing 
tube will experience equal and opposite tangential forces, of each 
of which the time-integral of the line-integral round the whole 
circumference is eAM, if the ring he a circle of radius r; and 
the effect of the generation of the magnetic field will he to cause 
the fluid and the ring to rotate in opposite directions with, moments 
of momentum each equal to eAMr, if neither fluid nor ring is acted 
on by any other force than that of the electromagnetic induction. 
Their angular velocities are therefore eAM/rw, eAM Jrw', and their 
kinetic energies are Je 2 A 2 M 2 /w, Je 2 A 2 M 2 /w', where w, w denote the 
masses of fluid and ring respectively. 
§ 3. Suppose now for simplicity in the first place, the ring to 
be embedded in ether, viewed as an incompressible solid, and 
