10 PROPERTIES OF AN ELECTRIC CHARGE [CH. i. 



where v is the speed of light and u is the acceleration 

 of the charge e. 



After this manner, though of course by means of 

 a very extensive development of these fundamental 

 ideas, are all the phenomena of electricity and mag- 

 netism and optics summarised, and, so to speak, 

 accounted for. 



NOTE. 



Let it be said here, once for all, that in every case one or other 

 of the two etherial constants, p and K, should be exhibited 

 explicitly wherever it rightly occurs. If this be not done the 

 dimensions of most expressions are necessarily wrong; and 

 words have to be added about whether the units intended are 

 c.g.s. or some other system, and likewise whether they are 

 electrostatic or electromagnetic. This latter double system of 

 measurement has served its turn, and still serves it ; but intrinsi- 

 cally it is confusing, and has been only rendered necessary 

 because we do not yet know the values, or even for certain the 

 t nature, of p and K. It is to be hoped that no third system 

 'devised as an attempted escape from confusion, but really an 

 intensification of fog will ever be successfully attempted ; though 

 there are threats in that direction, owing to lack of clear thinking. 



The explicit retention of the constants keeps everything clear 

 and easy. For instance, the expression just above quoted is 

 essentially what it purports to be, an energy divided by a time 



FL 



and is therefore true as it stands in every complete system of 

 units whatever. So also the expression quoted near the be- 

 ginning of the next chapter is essentially what it purports to be, 

 namely, a mass or inertia ; and the same may be said of all other 

 expressions in this book. 



The ordinary convention for numerical specification is, for 

 electromagnetic c.g.s. units, to consider /* = 1, for electrostatic 

 units, to consider K = I \ and this convention must hold until we 

 learn the real facts, by future discovery : for which discovery 

 continual familiarity with the unknown constants undoubtedly 

 serves to pave the way. 



