Page:Radio-active substances.djvu/47

 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 cm. in thickness, a current of magnitude 10-11 ampères is obtained, the rays utilised having traversed, before being absorbed by the disc, 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, ebonite and paraffin for the insulator; the results obtained were the same.

The current diminishes with increasing distance from the source of radiation,, also when a less active product is used.

We obtained the same results again when the disc 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.

Finally, we made the inverse experiment, which was to place the lead receptacle with the radium in the centre of

the insulating material and in connection with the electrometer (Fig. 7), the whole being surrounded with the metallic covering connected to earth.

Under these conditions, it is evident from the electrometer that the radium has a positive charge equal in magnitude to the negative charge of the former experiment. The radium rays penetrate the thin dielectric plate, p p, and leave the conductor inside carrying with them negative electricity.

The α-rays of radium do not interfere in these experiments, being almost completely absorbed by a very thin layer of matter. The method just described is not suitable for the study of the charge of the rays of polonium, these rays very slightly penetrating. We observed no indication of any charge in the case of polonium, which gives rise to α-rays only; but, for the reason just given, no conclusion can be drawn from this.