practical Instrument of Physical Research. 495 



particular form, interest such physicists as listen to the paper. 

 Prof. Poynting's theories of the transference of energy 

 through an electromagnetic field are to-day universally known. 

 In the Philosophical Transactions, 1884, part ii. pp. 343 to 

 349, he attempts to prove that the energy in the field has a 

 time-flux r given bj 



47tt=VEH, (41) 



where E and H are the electromotive and magnetic forces 

 respectively. In the Brit. Assoc. Reports, 1885, pp. 151, 

 152, Prof. J. J. Thomson points out that Prof. Poynting's 

 reasoning is open to criticism ; and that v, given by v — r + Vyo* 

 (where, if <r is discontinuous at any surface, the tangential 

 component is not so) will serve equally well, if assumed to be 

 the time-flux of energy, according to Prof. Poynting's reason- 

 ing, to explain the known facts. Put 47ro- = ^H, where "SP is 

 the electrostatic potential. [Note that, in order that this 

 value of a may satisfy the surface-conditions just mentioned, 

 M* must be continuous.] We get as the time-flux of energy, 



4,7rv = ±ttt + V y (¥H) = V (E + y¥) H + 4tt^C, 



where C, the current, is put in place of V\7H/47r. Substituting 

 from equation (10) § 599 of Maxwell's 'Electricity and 

 Magnetism,' viz. 



E = VGB-dA/d*-v% 



where G- is the velocity of matter at the point in question, we 

 see that for a steady field, 



u = ^C; (42) 



so that, assuming v instead of t to be the true time-flux of 

 energy, in this matter of the transference of energy through 

 the field, as in so many other respects, C the current and \P 

 the potential are the exact analogues of a liquid current and 

 the pressure. Compare this with the very different conclu- 

 sion of Prof. Poynting (Phil. Trans. 1884, part ii. p. 361:) — 

 " I think it is necessary that we should realize thoroughly 

 that, if we accept Maxwell's theory of energy residing in the 

 medium, we must no longer consider a current as something 

 conveying energy along the conductor." According to the 

 present result, in a steady field the sole means of conveyance 

 of energy would be precisely the means Prof. Poynting warns 

 us against, namely the electricity itself. 



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