e is at right angles to the direction of
projection of the particle, becomes a circle in the plane at right
angles to the magnetic force, the radius being mv/He, where m, v, e are
respectively the mass, velocity and charge on the particle, and H is the
magnetic force. The smaller the difference of potential between the
electrodes of the discharge tube the greater the deflection produced by
a magnetic field of given strength, and as the difference of potential
rapidly increases with diminution of pressure, after a certain pressure
has been passed, the higher the exhaustion of the tube the less the
magnetic deflection of the rays. Birkeland (_Comptes rendus_, 1896, p.
492) has shown that when the discharge is from an induction coil the
cathode rays produced in the tube at any one time are not equally
deflected by a magnet, but that a narrow patch of phosphorescence when
deflected by a magnet is split up into several distinct patches, giving
rise to what Birkeland calls the "magnetic spectrum." Strutt (_Phil.
Mag._ 48, p. 478) has shown that this magnetic spectrum does not occur
if the discharge of a large number of cells is employed instead of the
coil. Thomson (_Proc. Camb. Phil. Soc._ 9, p. 243) has shown that if the
potential difference between the electrodes is kept the same the
magnetic deflection is independent of the nature of the gas filling the
discharge tube; this was tested with gases so different as air,
hydrogen, carbonic acid and methyl iodide.

_Charge of Negative Electricity carried by the Rays._--We have seen that
the rays are deflected by a magnet, as if they were particles charged
with negative electricity. Perrin (_Comptes rendus_, 121, p. 1130)
showed by direct experiment that a stream of negative electricity is
associated with the rays. A modification made by Thomson of Perrin's
experiment is sketched in fig. 24 (_Phil. Mag._ 48, p. 478).

[Illustration: FIG. 24.]

The rays start from the cathode A, and pass through a slit in a solid
brass rod B fitting tightly into the neck of the tube. This rod is
connected with earth and used as the anode. The rays after passing
through the slit travel through the vessel C. D and E are two
insulated metal cylinders insulated from each other, and each having a
slit cut in its face so as to enable the rays to pass into the inside
of the inner cylinder, which is connected with an electrometer, the
outer cylinder being connected with the earth. The