Energy in an Interference Field. 

 Fin-. 1. 



21) 1 



Since j is perpendicular to kj and to k 2 the electric and 

 magnetic vectors at P have the following values*: 



— a cos n\ t — - ) 



■a cos n 



(•-?) 



— a cos 



E,= 



[kij], 



n ( £ — — ) — a cos nl t 2 ) 



_A !l/j, H 2 = V £_'[kj], 



where c is the velocity o£ light and a is a constant depending 

 on the charge of the vibrators and the amplitude of the 

 vibration. 



The flow of energy is determined by the Poynting vector 



On writing 



a cos nl t — — J = Cj, a cos nl t — — J = C 2 , 

 the equation 



is obtained. Now 



[j, [kj^k,, [j, [k 2 j]]=k 3 , 



* Terms containing higher powers of r t and r 2 in thp denominato]3 

 are disregarded. The formulas are in agreement with H. Hertz, Ann. 

 Phys. Chem. xxxvi. p. 1 (1888). See, e. a., Abraham, Theorie der Elek~ 

 trizitat, vol. ii. p. 62, or Lorentz, ' The Theory of Electrons,' p. 56. 



