falling to half value in about 1200 years. The period of movement of the leaf should thus gradually increase with the time, and ultimately the effect would become too small to observe.
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 Wien14. 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 electromagnetic units, this corresponds to an escape of 2·66 × 107 particles per second. From 1 gram of radium bromide the corresponding number would be 6·6 × 109