96 NATURE OF THE RADIATIONS [CH. 
showed, in addition, the deviation of the rays when a change 
occurred in the conductivity of the air under the influence of 
a magnetic field. Becquerel', a little later, showed the magnetic 
deflection of the radium rays by using the photographic method. 
P. Curie?, 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 a rays), and the 
other penetrating and deviable by a magnetic field (now known 
as the 8 rays). The ionization effect due to the @ rays was 
only a small fraction of that due to the a rays. At a later date 
Becquerel, by the photographic method, showed that uranium gave 
out some deflectable rays. It had been shown previously® that the 
rays from uranium consisted of « and 8 rays. The deflected rays 
in Becquerel’s experiment consisted entirely of @ rays, as the 
a rays from uranium produce no appreciable photographic action. 
Rutherford and Grier’, using the electric method, showed that 
compounds of thorium, like those of uranium, gave out beside 
a rays some penetrating 8 rays, deviable in a magnetic field. As 
in the case of radium, the ionization due to the a rays of uranium 
and thorium is large compared with that due to the £8 rays. 
70. Examination of the magnetic deviation by the 
photographic method. Becquerel has made a very complete 
study, by the photographic method, of the 8 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. J. J. Thomson (Recent Researches, p. 136) has 
obtained the equation for the path of a charged particle moving 
in a uniform magnetic field. If a particle of mass m and charge 
e is projected with a velocity V, at an angle « with the direction of 
a field of strength H, it will describe a curved path, whose radius 
R of curvature is given by 
The path of the particle is a helix wound on a cylinder of radius R 
with the axis parallel to the field. 
TAC ek. 1295 ppw9oi 205.) soo). 271 OR E30 S paiios G00: 
3 Rutherford, Phil. Mag. January, 1899. 4+ Phil. Mag. September, 1902. 
