showed, in addition, the deviation of the rays by the alteration of the conductivity of the air when a magnetic field was applied. Becquerel[1], a little later, showed the magnetic deflection of the radium rays by using the photographic method. P. Curie[2], by the electrical method, showed furthermore that the rays from radium consisted of two kinds, one apparently non-deviable and easily absorbed (now known as the α rays), and the other penetrating and deviable by a magnetic field (now known as the β rays). The ionization effect due to the β rays was only a small fraction of that due to the α rays. At a later date Becquerel, by the photographic method, showed that uranium gave out some deflectable rays. It had been shown previously[3] that the rays from uranium consisted of α and β rays. The deflected rays in Becquerel's experiment consisted entirely of β rays, as the α rays from uranium produce no appreciable photographic action. Rutherford and Grier[4], using the electric method, showed that compounds of thorium, like those of uranium, gave out, besides α rays, some penetrating β rays, deviable in a magnetic field. As in the case of radium, the ionization due to the α rays of uranium and thorium is large compared with that due to the β rays.
76. Examination of the magnetic deviation by the photographic method. Becquerel has made a very complete
study, by the photographic method, of the β rays from radium,
and has shown that they behave in all respects like cathode rays,
which are known to be negatively charged particles moving with
a high velocity. The motion of a charged ion acted on by a
magnetic field has been discussed in section 49. It has been
shown that if a particle of mass m and charge e is projected
with a velocity u, at an angle α with the direction of a uniform
field of strength H, it will describe a helix round the magnetic
lines of force. This helix is wound on a cylinder of radius R, with
the axis parallel to the field, where R is given by
R = (mu/He) sin α.
