=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 _mu_ _R_ = ---- sin α. _He_ When α = π/2, _i.e._ when the rays are projected normally to the field, the particles describe circles of radius _mu_ _R_ = ---- _He_ The planes of these circles are normal to the field. Thus, for a particular velocity _u_, the value of _R_ varies inversely as the strength of the field. In a uniform field the rays projected normally to the field describe circles, and their directions of projection are the tangents at the origin. This conclusion has been verified experimentally by Becquerel for the β rays of radium, by an arrangement similar to that shown in Fig. 23. [Illustration: Fig. 23.] A photographic plate _P_, with the film downwards, is enveloped in black paper and placed horizontally in the uniform horizontal magnetic field of an electromagnet. The magnetic field is supposed to be uniform, and, in the figure, is at right angles to the plane of the paper. The plate was covered with a sheet of lead, and on the edge of the plate, in the centre of the magnetic field, is placed a small lead vessel _R_ containing the radio-active matter. On exciting the magnet, so that the rays are bent to the left of the figure, it is observed that a photographic impression is produced directly below the source of the rays, which have been bent round by the magnetic field. The active matter sends out rays equally in all directions. The rays perpendicular to the field describe circles, which strike the plate immediately under the source. A few of these rays, _A₁_, _A₂_, _A₃_, are shown in the figure. The rays, normal to the plate, strike the plate almost normally, while the rays nearly parallel to the plate strike the plate at grazing incidence. The rays, inclined to the direction of the field, describe spirals and produce effects on an axis parallel to the field passing through the source. In consequence of this, any opaque screen placed in the path of the rays has its shadow thrown near the edge of the photographic plate.