Energy in the Elect rodynamic Field. 389 



Thus we have 



dT 



dt 



so that if we take 



+ i s "" /= ^i) EH] " + ^i( H ^) rfc; 



we might also assume 



S = £[EH], 



which is Pointing's vector. Irs dependence on the parti- 

 cular form for T is clearly indicated. 



3. There is, however, no definite and precise reason 

 why we should transform the volume integral in this way. 

 We might, for instance, introduce the vector and scalar 

 potentials A, <\> by the substitution 



we should then have 



j m w>*--ij;(b^)*-j;(ov)#* 



and since .. _. 



div C = 



under all circumstances, this leads to the relation 



Thus if we now take 



T =i(rf»f (CdA), 



we could assume 



8 = 00; 



and this appears to be the proper result in the simplest and 

 most general form of Macdonald's theory. It differs 

 markedly from his result however, for a reason which 

 will subsequently appear. 



