280 Professors Ayrton and Perry on the Ratio of the 



same constancy as a resistance-coil*; therefore, if any one 

 desires to measure with great accuracy the electromotive force 

 of his battery, he is not able to do this by a simple comparison 

 with a standard cell, but he must determine it absolutely him- 

 self. Now the simplest way to do this, is to measure its 

 electrostatic value with an absolute electrometer, and to con- 

 vert the result into electromagnetic measure or into volts, by 

 using the proper multiplier, which necessarily depends on the 

 ratio v. 



There are also other practical uses that may be made of 

 this constant ; but the main interest attached to the exact de- 

 termination of its value consists in its constituting a test of 

 the accuracy of the theory that the same medium which 

 transmits the vibrations that constitute light transmits those 

 also which produce electro-magnetic induction. 



For the two units of electric quantity are of a totally 

 different nature from one another. If, for example, you take 

 a yard and a foot, two units of length, and divide the one by 

 the other, you get the simple number three, or, if you take 

 two units of weight, a ton and a pound, you get a simple 

 number 2240 ; but if you divide the electromagnetic by the 

 electrostatic unit of quantity, it is more like dividing a solid 

 by an area : in that case you do not get for your quotient a 

 number, or a solid, or an area, but a length. So the ratio of the 

 electric units is not a number but a velocity f , and an absolute 

 velocity in nature independent of the units of space and time ; 

 and Prof. Clerk Maxwell has proved that this velocity must 

 be that of the propagation of electromagnetic disturbances in 

 a non-conducting medium, or, assuming that light is an 

 electromagnetic disturbance, must be equal to the velocity of 



light t. 



1. Previous Measurements of "v." 



The first estimate of the relation between a quantity of 

 electricity measured statically and the quantity transferred by 



* The electromotive force of Mr. Latimer Clark's mercurous sulphate 

 cell is undoubtedly very constant, but is necessarily altered by the shaking- 

 in travelling, by the presence, or absence, of free mercury in the paste, by 

 the mode in which the mercurous sulphate paste is prepared, &c. In fact, 

 as we have pointed out (Proc. Roy. Soc. No. 186, 1878), measurements of 

 electromotive force are far more delicate than any chemical tests. 



t The dimensions of a quantity of electricity measured electrostatically 



are 2 '"- 2 > measured electro-magnetically [jJ M?] ; therefore the di- 

 mensions of the ratio is ^ a velocity. (Jenkin, ' Electricity and Mag- 

 netism,' p. 164.) 



X ' Electricity and Magnetism/ chapter xx. 



