342 Prof. J. J. Thomson on 
by the magnetic field, so that a given impulse will produce 
the same velocity in the particle whatever the magnitude of 
the field. The magnetic field affects.the subsequent motion 
of the particle, but not its initial velocity or the pulses 
produced when it is started. 
As another example of the principle of the Conservation of 
Momentum, let us consider the case where the electrified 
particle is suddenly started with a velocity w parallel to the 
axis of ¢ in a uniform electric field parallel to w, the field 
being supposed to be due to uniformly electrified plates 
parallel to yz. If we calculate by the rule given above the 
art of the momentum arising from the electric field, we 
shall find that while the momentum in any direction vanishes 
the moment of momentum is finite. 
If X is the electric force, K and w the specific inductive 
capacity and magnetic permeability of the dielectric, then 
the moment of momentum about the axis of y in the region 
inside the pulse is equal to 
BK Xew 
2a! 
é (OB? —a’), 
where OB is the inner radius of the pulse; it is equal 
to Vt, where V is the velocity of light and ¢ the interval 
which has elapsed since the particle was started. 
The moment of momentum in the pulse itself is equal to 
wK . Xew 
3 
hence, the total moment of momentum about y is equal 
to 
OP? ; 
a OB?—4yK . Xewa? 
=} yk. XewV???—1 pK . Xewa’. 
. ; aaa dy 
'The rate of increase of this, since wK= yp is Xewt. Thus to. 
keep the moment of momentum of the whole system constant, 
the material parts of the system, i. e. the charged point and 
the plates, must be acted on by a couple about the axis of y 
whose moment is —Xewt. We can see that this is the case, 
for suppose O is the position of the charged body before it is 
set in motion, O’ the position after a time ¢, then, when the 
particle was at O the force on the plates was equal and oppo- 
site to that on the particle, it therefore passed through O and 
was equal to Xe. When, however, the particle is set in 
