=89. Magnetic deviation of the α rays=. The general method employed[141] to detect the magnetic deviation of the α rays was to allow the rays to pass through narrow slits and to observe whether the rate of discharge of an electroscope, due to the issuing rays, was altered by the application of a strong magnetic field. Fig. 32 shows the general arrangement of the experiment. The rays from a thin layer of radium of activity 19,000 passed upwards through a number of narrow slits _G_, in parallel, and then through a thin layer of aluminium foil, ·00034 cm. thick, into the testing vessel _V_. The ionization produced by the rays in the testing vessel was measured by the rate of movement of the leaves of a gold-leaf electroscope _B_. The gold-leaf system was insulated inside the vessel by a sulphur bead _C_, and could be charged by means of a movable wire _D_, which was afterwards earthed. The rate of movement of the gold-leaf was observed through small mica windows in the testing vessel by means of a microscope provided with a micrometer eye-piece. [Illustration: Fig. 32.] In order to increase the ionization in the testing vessel, the rays passed through 20 to 25 slits of equal width, placed side by side. This was arranged by cutting grooves at regular intervals in side-plates into which brass plates were slipped. The width of the slit varied in different experiments between ·042 cm. and ·1 cm. The magnetic field was applied perpendicular to the plane of the paper, and parallel to the plane of the slits. The rays are thus deflected in a direction perpendicular to the plane of the slits and a very small amount of deviation is sufficient to cause the rays to impinge on the sides of the plate where they are absorbed.