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[1]. 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[2] and Becquerel[3] 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
