Electromagnetic to the Electrostatic Unit of Electric Quantity. 281 



a current in a given time was made by Faraday * ; but being 

 measured in arbitrary units, as the absolute system was not 

 then developed, Faraday's comparison gave no indication of 

 the value of v. 



There are several ways of measuring this value. The first 

 numerical determination was made, in 1856, by Messrs. Weber 

 and Kohlrauschf; and their method was founded on the 

 measurement of the same quantity of electricity, first in 

 electrostatic and then in electromagnetic units. The result 

 they obtained was 3 10' 7 million metres per second. 



But as the quantity they measured electrostatically was prac- 

 tically the amount discharged in a finite time by a Leyden jar 

 previously electrified to a fixed difference of potentials, and 

 the amount measured electromagnetically was the instan- 

 taneous discharge of the same Leyden jar electrified to the 

 same difference of potentials, it is probable that the result of 

 the first of these measurements and consequent value of v ob- 

 tained was, on account of the electric absorption of the glass, 

 rather too large. 



The next determination of the value of v was made by Sir 

 W. Thomson in 1868 J, who measured the same electro- 

 motive force electrostatically with his absolute electrometer, 

 and electromagnetically by determining with an electro- 

 dynamometer the electromagnetic value of the current sent 

 by this electromotive force through a known resistance. The 

 mean of eleven sets of experiments, from which the highest 

 value obtained was 292 and the lowest 275, gave a result of 

 282*5 million metres per second. 



In the preceding method two forces had to be separately 

 measured, one by means of an electrometer, and the other 

 with an electrodynamometer ; but Prof. Clerk Maxwell, about 

 the same time, carried out a method by means of which 

 these two forces were made to balance one another, so that 

 the ratio of the electrostatic and electromagnetic measures of 

 the same electromotive force was obtained without previously 

 ascertaining the value of each. The highest of the twelve 

 most accurate results was 294, and the lowest 284, and the 

 mean for v 288 million metres per second. In both these 

 methods it was necessary to know the absolute resistance of a 

 certain coil employed. 



It will be observed that even the highest values obtained 

 by either Sir Wm. Thomson or Prof. C. Maxwell were lower 



* Experimental Researches, Series iii. § 362. 

 t C. Maxwell, 'Electricity and Magnetism,' ch. xix. p. 370. 

 % Sixth Report of the Committee of the British Association on Elec- 

 trical Standards, 1869. 



