According to the preceding experiments, it was evident that the charge of the rays of the radiating body employed was a weak one.
In order to fix a feeble evolution of electricity upon the conductor which absorbs the rays, this conductor should be completely insulated; this is effected by screening it from the air, either by placing it in a tube with a very perfect vacuum, or by surrounding it with a good solid dielectric. We employed the latter arrangement.
Fig. 6.
A conducting disc, M M (Fig. 6), is connected by the wire, t, to the electrometer; the disc and wire are completely enveloped by the insulating substance i i i i; the whole is again surrounded with the metallic covering, E E E E, which is in electric connection with the earth. The insulator, p p, and the metallic envelope are very thin upon one of the faces of the disc. This face is exposed to the radiation of the barium and radium salt, R, placed outside in a lead receptacle. The rays emitted by the radium penetrate the metallic envelope and the insulating lamina, p p, and are absorbed by the metallic disc, M M. The latter then becomes the source of a continuous evolution of negative electricity, as determined by the electrometer, and is measured by means of a quartz piezometer.
The current thus created is very weak. With very active barium-radium chloride, forming a layer of 2·5 sq. c.m. in area, and of 0·2 c.m. in thickness, a current of magnitude 10–11 ampères is obtained, the rays utilised having traversed, before being absorbed by the disc M M, a thickness of aluminium of 0·01 m.m., and a thickness of ebonite of 0·3 m.m.
We used successively lead, copper, and zinc for the disc M M, ebonite and paraffin for the insulator; the results obtained were the same.
The current diminishes with increasing distance from the source of radiation, R, also when a less active product is used.
We obtained the same results again when the disc M M is replaced by a Faraday cylinder filled with air, and covered outside with insulating material. The opening of the cylinder, closed by the thin insulating plate, p p, was opposite the radiating source.
