Middle of the Nineteenth Century. 263 



would appear on a body turning with the circuit as if 

 rigidly connected with it. Again,* 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 Kiemann'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 Frohlichf 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 BuddeJ 

 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. 



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 crudal experiments, with others, were suggested by 

 E. Budde, Ann. d. Phys. xxx (1887), p. 100. 



t Ann. d. Phys. ix (1880), p. 261. 



+ Ann. d. Phys. x (1880), p. 553. 



This case of a charge and current moving side hy side was afterwards 

 examined by Fitz Gerald (Trans. Boy. Dub. Soc. i, 1882 ; Scient. Writings of 

 G. F. Fitz Gerald, p. Ill) 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. 



