360 
PROFESSOR J. H. POYNTING- ON THE TRANSFER 
This at once gives us the magnetic equal to the electric energy, for 
/4> 2 _ fiKH-d? _ KW 
87 r 87r 87 t 
It may be noted that the velocity ^ is the greatest velocity with which the two 
energies can be propagated together, and that they must be equal when travelling 
with this velocity. For if v be the velocity of propagation and 6 the angle between 
the two intensities, we have 
47rr 87 t 87t 
or 
2 sin 6 
•§ + e 
% 
The greatest value of the numerator is 2 when 6 is a right angle, and the least 
value of the denominator is 2\ /xK, when the two terms are equal to each other and 
to \/ g K. 
The maximum value of v therefore is an d occurs when #= — and K6'=/x^'l 
The preceding examples will suffice to show that it is easy to arrange some of the 
known experimental facts in accordance with the general law of the flow of energy. I 
am not sure that there has hitherto been any distinct theory of the way in which the 
energy developed in various parts of the circuit has found its way thither, but there 
is, I believe, a prevailing and somewhat vague opinion that in some way it has been 
carried along the conductor by the current. Probably Maxwell’s use of the term 
“ displacement” to describe one of the factors of the electric energy of the medium has 
tended to support this notion. It is very difficult to keep clearly in mind that this 
“ displacement” is, as far as we are yet warranted in describing it, merely a something 
with direction which has some of the properties of an actual displacement in incom¬ 
pressible fluids or solids. When we learn that the “ displacement” in a conductor 
having a current in it increases continually with the time, it is almost impossible to 
avoid picturing something moving along the conductor, and it then seems only natural 
to endow 7 this something with energy-carrying power. Of course it may turn out that 
there is an actual displacement along the lines of electromotive intensity. But it is 
quite as likely that the electric “ displacement” is only a function of the true displace¬ 
ment, and it is conceivable that many theories may be formed in which this is the 
case, while they may all account for the observed facts. Mr. Glazebrook has already 
worked out one such theory in which the component of the electric displacement at 
