0, and moving towards the plane x = a. The particles starting from x =
0 describe cycloids, and the greatest distance they can get from the
plane is equal to the diameter of the generating circle of the
cycloid, i.e. to 2Xm/eH^2. (After reaching this distance they begin to
approach the plane.) Hence if a is less than the diameter of the
generating circle, all the particles starting from x = 0 will reach
the plane x = a, if this is unlimited in extent; while if a is greater
than the diameter of the generating circle none of the particles which
start from x = 0 will reach the plane x = a. Thus, if x = 0 is a plane
illuminated by ultra-violet light, and consequently the seat of a
supply of negative ions, and x = a a plane connected with an
electrometer, then if a definite electric intensity is established
between the planes, i.e. if X be fixed, so that the rate of emission
of negative ions from the illuminated plate is given, and if a is less
than 2Xm/eH^2, all the ions which start from x = 0 will reach x = a.
That is, the rate at which this plane receives an electric charge
will be the same whether there is a magnetic field between the plate
or not, but if a is greater than 2Xm/eH^2, then no particle which
starts from the plate x = 0 will reach the plate x = a, and this plate
will receive no charge. Thus the supply of electricity to the plate
has been entirely stopped by the magnetic field. Thus, on this theory,
if the distance between the plates is less than a certain value, the
magnetic force should produce no effect on the rate at which the
electrometer plate receives a charge, while if the distance is greater
than this value the magnetic force would completely stop the supply of
electricity to the plate. The actual phenomena are not so abrupt as
this theory indicates. We find that when the plates are very near
together the magnetic force produces a very slight effect, and this an
increase in the rate of charging of the plate. On increasing the
distance we come to a stage where the magnetic force produces a great
diminution in the rate of charging. It does not, however, stop it
abruptly, there being a considerable range of distance, in which the
magnetic force diminishes but does not destroy the current. At still
greater distances the current to the plate under the magnetic force is
quite inappreciable compared with that when there is no magnetic
for