would appear on a body turning with the circuit as if rigidly connected with it. Again,[1] let a magnet be suspended within a hollow metallic body, and let the hollow body be suddenly charged or discharged; then, according to Clausius' theory, the magnet is unaffected; but according to Weber's and Riemann's theories it experiences an impulsive couple. And again, if an electrified disk be rotated in its own plane, under certain circumstances a steady current will be induced in a neighbouring circuit according to Weber's law, but not according to the other formulae.
An interesting objection to Clausius' theory was brought forward in 1879 by Fröhlich[2]—namely, that when a charge of free electricity and a constant electric current are at rest relatively to each other, but partake together of the translatory motion of the earth in space, a force should act between them if Clausius' law were true. It was, however, shown by Budde[3] that the circuit itself acquires an electrostatic charge, partly as a result of the same action which causes the force on the external conductor, and partly as a result of electrostatic induction by the charge on the external conductor; and that the total force between the circuit and external conductor is thus reduced to zero.[4]
We have seen that the discrimination between the different laws of electrodynamic force is closely connected with the question whether in an electric current there are two kinds of electricity moving in opposite directions, or only one kind moving in one direction. On the unitary hypothesis, that the
- ↑ The two following crucial experiments, with others, were suggested by E. Budde, Ann, d. Phys, xxx (1887), p. 100.
- ↑ Ann. d. Phys. ix (1880), p. 261.
- ↑ Ann. d. Phys. x (1880), p. 553.
- ↑ This case of a charge and current moving side by side was afterwards examined by FitzGerald (Trans. Roy. Dub. Soc. i, 1882; Scient. Writings of G. F. FitzGerald, p. 111) without reference to Clausius' formula, from the standpoint of Maxwell's theory. The result obtained was the same—namely, that the electricity induced on the conductor carrying the current neutralizes the ponderomotive force between the current and the external charge.
