When α = π/2, i.e. when the rays are projected normally to the field, the particles describe circles of radius
R = mu/(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.
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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, A_{1}, A_{2}, A_{3}, are shown in the figure. The rays, normal to the plate, strike the plate almost normally,
