radium. No alteration is observed if the radium is heated in a closed vessel from which none of the radio-active products are able to escape.
122. Motion of radium in an electric field. Joly[1] found
that a disc, one side of which is coated with a few milligrams of
radium bromide, exhibits, when an electrified body is brought
near it, motions very different to those observed in the case of
an inactive substance. The electrified body, whether positive or
negative, repels the suspended body if brought up to it on the
side coated with radium, but attracts it if presented to the naked
side.
This effect is very simply shown by constructing a small apparatus like a radiometer. Two covered glasses are attached to the end of a glass fibre about 6 cms. long, the surfaces lying in the same plane. The apparatus is free to rotate on a pivot. The two vanes are coated on alternate faces with radium bromide, and the whole apparatus contained within a glass receiver. If an electrified rod of ebonite or sealing wax is brought up close to the receiver, a rotation is communicated to the vane which increases as the pressure of the air is lowered to 5 or 6 cms. of mercury. By placing the apparatus between parallel plates connected with the terminals of a Wimshurst machine, a steady rotation is communicated to the vanes. The rotation is always in such a direction that the radium coated surface is repelled from the electrified body.
This action was examined still further by attaching the vanes to the glass beam of a Coulomb's balance. A metal sphere, which could be charged from without, was fixed facing the side coated with radium. A repulsion was always observed except when the charge was very strong and the vane near the sphere. If, however, the two vanes were connected by a light wire and a similar sphere placed exactly opposite the other, an attraction was observed if one sphere was charged, but a repulsion if both were charged. These effects were observed whether the vanes were of aluminium or glass.
Joly found that the effect could not be explained by any direct
- ↑ Joly, Phil. Mag. March, 1904.
