Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/294

he connexion between the strength ι of a current and the magnetic field H, to which it gives rise, may be represented by the equation

$4\pi \iota =\mathrm {curl} \ \mathbf {H}$;

this equation is equivalent to the statement that "the entire magnetic intensity round the boundary of any surface measures the quantity of electric current which passes through that surface."

In the same year (1856) in which Maxwell's investigation was published, Thomson^[1] put forward an alternative interpretation of magnetism. He had now come to the conclusion, from a study of the magnetic rotation of the plane of polarization of light, that magnetism possesses a rotatory character: and suggested that the resultant angular momentum of the thermal motions of a body^[2] might be taken as the measure of the magnetic moment. "Tho explanation," he wrote, "of all phenomena of electromagnetic attraction or repulsion, or of electromagnetic induction, is to be looked for simply in the inertia or pressure of the matter of which the motions constitute heat. Whether this matter is or is not electricity, whether it is a continuous fluid interpermeating the spaces between molecular nuclei, or is itself molecularly grouped: or whether all matter is continuous, and molecular heterogeneousness consists in finite vortical or other relative motions of contiguous parts of a body: it is impossible to decide, and, perhaps, in vain to speculate, in the present state of science."

The two interpretations of magnetism, in which the linear and rotatory characters respectively are attributed to it, occur frequently in the subsequent history of the subject. The former was amplified in 1858, when Helmholtz published his researches^[3] on vortex motion; for Helmholtz showed that if a

1. Proc. Roy. Soc. viii (1856), p. 150; xi (1861), p. 827, foot-note: Phil. Mag. xiii (1867), p. 198; Baltimore Lectures, Appendix F.
2. This was written shortly before the kinetic theory of gases was developed by Clausius and Maxwell.
3. Journal für Math. lv (1858), p. 25; Helmholtz's Wiss. Abh, i, p. 101; translated Phil. Mag. xxxiii (1867), p. 485.