Page:Radio-activity.djvu/144

 The action of this radium clock is the nearest approach to an apparent perpetual motion that has so far been observed.

A determination of the amount of the charge carried off by the β rays of radium has been made by Wien. A small quantity of radium, placed in a sealed platinum vessel, was hung by an insulating thread inside a glass cylinder, which was exhausted to a low pressure. A connection between the platinum vessel and an electrode sealed on to the external glass cylinder could be made, when required, by tilting the tube. Wien found that in a good vacuum the platinum vessel became charged to about 100 volts. The rate of escape of negative electricity from the platinum vessel containing 4 milligrams of radium bromide corresponded to 2·91 × 10^{-12} amperes. If the charge on each particle is taken as 1·1 × 10^{-20} electro-magnetic units, this corresponds to an escape of 2·66 × 10^7 particles per second. From 1 gram of radium bromide the corresponding number would be 6·6 × 10^9 per second. Since some of the β rays are absorbed in their passage through the walls of the containing vessel and through the radium itself, the actual number projected per second from 1 gram of radium bromide must be greater than the above value. This has been found by the writer to be the case. The method employed reduced the absorption of the β rays to a minimum, and the total number emitted per second by 1 gram of radium bromide in radio-active equilibrium was found to be 4·1 × 10^{10}, or about six times the number found by Wien. A detailed account of the method employed cannot be given with advantage at this stage, but will be found later in Section 246.

81. Determination of e/m. We have seen (Section 50) that, in their passage between the plates of a condenser, the cathode rays are deflected towards the positive plate. Shortly after the discovery of the magnetic deviation of the β rays from radium, Dorn and Becquerel showed that they also were deflected by an electric field.

By observing separately the amount of the electric and magnetic deviation, Becquerel was able to determine the ratio of e/m and the velocity of the projected particles. Two rectangular copper