1873.] 



Units in the Electromagnetic Unit. 



291 



The two systems of electrical measurement, or the units which they 

 employ, are founded on the fundamental units of time, mass, and space 

 applied to the observed effects of electricity at rest and electricity in 

 motion. The dimensions of quantity in the two systems are such that 

 the ratio of the electromagnetic and the electrostatic unit of quantity is 

 expressible as a velocity. 



This velocity, usually known as v, is not only of great importance 

 in all combinations of electromagnetic and electrostatic action, but it is 

 also of great scientific importance in the theory of the propagation of 

 electromagnetic disturbances through a dielectric medium. It occupies a 

 very important place in the development of the electromagnetic theory 

 of light by Professor Clerk Maxwell, according to whose theory this 

 velocity v is the same as the velocity of light. 



The first experimental determination of v was made by "Weber from a 

 common electrostatic and electromagnetic measure of capacity. As the 

 result of Weber's experiments, v was found to be 310*74 x 10 s centims. 

 per second. 



Another determination was made by Prof. Clerk Maxwell in 1868, by 

 means of a direct comparison of electrostatic attraction with electro- 

 magnetic repulsion. His experiments gave v = 288xl0 8 centims. per 

 second. 



The value of v given by the experiments here described is 293 x 10 s 

 centims. per second. The method employed was that of obtaining an 

 absolute electrostatic and an absolute electromagnetic measurement of 

 the same electromotive force, v is defined as the ratio of the units of 

 quantity in the two systems ; but it follows from the definition of electro- 

 motive force, that v is also the ratio of the units of electromotive force in 

 the two systems. 



The electromotive force, or the difference of potentials between the two 

 poles of a constant Daniell's battery, was measured electrostatically by 

 means of Sir "William Thomson's absolute electrometer. The absolute 

 electromagnetic value of this electromotive force was given by the effect 

 of the current which it maintained in the circuit of an electrodynamo- 

 meter. The determination of this value depended on the resistance of the 

 electrodynamometer-circuit, which was reckoned in terms of the absolute 

 value of the British-Association standard unit of resistance. Any cor- 

 rection which may hereafter be found to be applicable to the absolute 

 value of this standard coil, as measured at King's College by Professors 

 Clerk Maxwell, Balfour Stewart, and Pleeming Jenkin, must be applied 

 to the value of v given above. 



The comparisons made in 1SG7 and 1868 by Mr. King gave as the 

 mean value of v, 284*6 x 10 8 centims. per second. 



The experiments made in 1870 with the new absolute electrometer 

 gave as the mean result v =294'5 x 10 8 centims. per second. The result 



VOL. XXI. 2 B 



