SECTION A. 
38 PRESIDENTS ADDRESS 
per unit volume of a field traversed by electromagnetic distur- 
eae Suna et. . oe : 
bances 1s +i if the intensities may be taken as uni- 
8 xz 8 z 
form through the small volume considered. Now, if the waves 
pass on unchanged in form with uniform velocity, then the energy 
in any part of the system may also be considered to pass on 
unchanged with the same velocity. Let the velocity be V, then 
the energy contained in a unit volume of cubical form with a face 
in the wave front will all pass through that face in one V“ of a 
second. Now the directions of the electric and magnetic intensi- 
ties are by the principles of the theory at right angles to one 
another in a homogeneous non-magnetisable medium, and the 
direction of both must be normal to the direction of propagation, 
both from what has been said as to the sideway motion of the 
energy, and from a direct calculation by Maxwell (Vol. IT, p. 400). 
Let us suppose that the direction of propogation is parallel to the 
axes of z; the electric polarisation will be, say, up and down, 
while the magnetic intensity is right and left. The rate at which 
energy may move in a magnetic field has been shown from 
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Maxwell’s equations to be ye ee second. This statement is a 
T 
précis of Poynting’s deduction, and is a possible solution whether 
we are considering the energy of a wave motion or the passage of 
the energy of a strained dielectric into a wire. The proof is to be 
found in Poynting’s paper, Phil. Trans. 1884 ,; it is too long to 
reproduce here, and, though sufticient for our purpose, has met 
with criticism. The quantity of energy, therefore, which passes 
out through the side of the cube in 1/V seconds must be ra 
and this must be the whole energy of the cube: so we have 
EH KE? H? 
AO ATT eng f 8 7 
pencicular to the direction of the polarisation, and hence containing 
the magnetic intensity, we know by one of the principles of the 
theory that the magnetomotive force round the face must be 
equal to 4 7 x current through the face. 
Now this may be written in terms of the distance from some 
fixed plane along the direction of propagation, as a function of z 
in fact, so the magneto-motive force round the face may be put 
OY lew bie Alert iia! Se Aaa qv x 7@e& 
ees while the current is ree so that — re = 
sut since the displacement is propagated onward with velocity 
V: after atime d¢ the displacement at any point will become 
replaced by one which was at a distance dz or Vat behind, so 
a— ai 
aeé dot 
that ae = -V —— substituting for above we! ret) 
as at az 
Taking a face of the cube ‘per- 
