Momentum in the Electric Field. 355 
and motion of the point itself, there is a force acting on wit 
electric charge, 7. e. an electric force equal to 
DE fa feemnee) iy, (EH) | 
these are the usual expressions for electromotive forces due 
os 
é da; 
the term derived from the scalar potential. 
to the induction of current, the term being included in 
On the Momentum in Electrical Waves. 
Pressure of Radiation. 
Let us consider a plane electrical wave in which, as we 
shall prove later, the electric force and the magnetic force 
are at right angles to each other, and both in the wave-front. 
Let H be the magnetic force at a point in the wave-tront, 
then X, the electric force, is in a progressive wave equal toVH, 
where V is the velocity of light, X and H being measured in 
electromagnetic units ; the electric polarization parallel to X 
is therefore H/47rV, so that the momentum which is at right 
angles both to F and H is along the normal to the wave-. 
front, 7. e. in the direction of propagation of the wave, and is 
2 
so Now H?/47 is the energy 
(electrostatic + electromagnetic) per unit volume, so that the 
momentum per unit volume is equal to (energy per unit 
volume)/(velocity of propagation of the wave). If this wave 
were to fall on a body by which it is absorbed without 
reflexion, this momentum would be communicated to the 
absorbing body, the amount of momentum communicated to. 
the body per unit time would be per unit area of wave-front 
equal to V times the momentum per unit volume; it would 
therefore equal the energy per unit volume in the wave. 
This communication of momentum to the body being pro- 
portional to the area of the surface, might be produced by the 
application of a suitable pressure to the surface, the pressure 
being equal to the momentum communicated to the body per 
unit area per unit time. Thus a body absorbing electrical 
waves behaves as if a pressure acted on its surface in the 
direction of propagation of the wave equal to the energy per 
unit volume in the wave. Light waves are a particular case 
of electrical waves, hence when light falls on an absorbing 
substance it exerts a pressure on the surface equal to the 
energy per unit volume in the light wave. The existence of 
equal per unit volume to 
