Page:Radio-activity.djvu/165

 The mica plate, ·01 cm. thick, was of sufficient thickness to absorb completely all the α rays, while it allowed the β rays and γ rays to pass through without appreciable absorption. The difference between (1) and (3), 7·40 volts per minute, gives the rate of discharge due to the [Greek: alpha] rays alone; the difference between (2) and (3), 0·79 volts per minute, that due to the α rays not deviated by the magnetic field employed.

The amount of [Greek: alpha] rays not deviated by the field is thus about 11% of the total. The small difference between (3) and (4) measures the small ionization due to the [Greek: beta] rays, for they would be completely deviated by the magnetic field; (4) comprises the effect of the [Greek: gamma] rays together with the natural leak of the electroscope in hydrogen.

In this experiment there was a good deal of stray magnetic field acting on the rays before they reached the pole-pieces. The diminution of the rate of discharge due to the [Greek: alpha] rays was found to be proportional to the strength of field between the pole-pieces. With a more powerful magnetic field, the whole of the α rays were deviated, showing that they consisted entirely of projected charged particles.

In order to determine the direction of deviation of the rays, the rays were passed through slits one mm. in width, each of which was half covered with a brass strip. The diminution of the rate of discharge in the testing vessel for a given magnetic field in such a case depends upon the direction of the field. In this way it was found that the rays were deviated in the opposite sense to the cathode rays. Since the latter consist of negatively charged particles, the [Greek: alpha] rays must consist of positively charged particles.

These results were soon after confirmed by Becquerel, by the photographic method, which is very well adapted to determine the character of the path of the rays acted on by a magnetic field. The radium was placed in a linear groove cut in a small block of lead. Above this source, at a distance of about 1 centimetre, was placed a metallic screen, formed of two plates, leaving between them a narrow opening parallel to the groove. Above this was placed the photographic plate. The whole apparatus was placed in a strong magnetic field parallel to the groove. The strength of the