580-582] Units 507 



Out of this suggestion, amply borne out by the results of further experi- 

 ments, has grown the Electromagnetic Theory of Light, of which a short 

 account will be given in the next chapter. From an examination of different 

 experimental results, Cornu* gives as the most probable value of the velocity 

 of light in free ether 



3-0013 '0027 x 10 10 cms. per second. 



Dividing by 1 '000294, the refractive index for light passing from a vacuum 

 to air, we find as the velocity of light in air, 



3'0004 + -0027 x 10 10 cms. per second. 



This quantity, again, is identical, except for a difference which is well 

 within the limits of experimental error, with the quantities already obtained. 



Thus we may say that the ratio of units C is identical with the velocity 

 of propagation of electromagnetic waves, and this again is identical with the 

 velocity of light. 



UNITS. 



582. We can at this stage sum up all that has been said about the 

 different systems of electrical units. 



There are three different systems of units to be considered, of which two 

 are theoretical systems, the electrostatic and the electromagnetic, while the 

 third is the practical system. We shall begin by discussing the two 

 theoretical systems and their relation to one another. 



In the Electrostatic System the fundamental unit is the unit of electric 

 charge, this being defined as a charge such that two such charges at unit 

 distance apart in air exert unit force upon one another. There will, of 

 course, be different systems of electrostatic units corresponding to different 

 units of length, mass and time, but the only system which need be considered 

 is that in which these units are taken to be the centimetre, gramme and 

 second respectively. 



In the Electromagnetic System the fundamental unit is the unit mag- 

 netic pole, this being defined to be such that two such poles at unit distance 

 apart in air exert unit force upon one another. Again the only system 

 which need be considered is that in which the units of length, mass and 

 time are the centimetre, gramme and second. 



From the unit of electric charge can be derived other units e.g. of 

 electric force, of electric potential, of electric current, etc., in which to 

 measure quantities which occur in electric phenomena. These units will 

 of course also be electrostatic units, being derived from the fundamental 

 electrostatic unit. 



* i.e. p. 246. 



