ELECTRICAL LEVEL OR POTENTIAL 79 



If this volume contains half a unit of energy, the quantity per 

 unit volume 



J_ E 2 



47T STT 

 E 2 



or, substituting for E its equivalent 47rD, 



Now this result is of the kind which we should expect from 

 Faraday's view of the nature of electric action. For if electric 

 strain be analogous to elastic strain, we should expect that the 

 energy stored up per unit volume would be proportional to the 

 square of the electric strain, just as the energy stored by an elastic 

 strain for which Hooke's law holds is proportional to the square of 

 the strain. Since then the distribution at the rate of half a unit 



E 2 



to each cell or of = 2?rD 2 to each unit volume just accounts for 

 OTT 



the energy of the system, and also carries further the analogy 

 between electric and elastic strain, it is accepted as the true law of 

 distribution. 



E 2 

 It is noteworthy that the energy per unit volume - may be 



O7T 



expressed in the form -^-. This supports the suggestion on p. 101 



that the intensity E is analogous to elastic stress. For we know 

 that in elastic strain 



stress x strain 

 energy per unit volume = - ^ - . 



