Page:Radio-activity.djvu/173

 the two parallel plates, placed about 3 mms. apart, was initially 6·5 × 10^{-9} amperes and fell off directly as the pressure. The current reached a limiting value of about 6 × 10^{-12} amperes, or about 1/1000 of the value at atmospheric pressure. The magnitude of this limiting current was not much altered if the air was replaced by hydrogen.

Experiments of a similar character have been made by Strutt and J. J. Thomson ; using an active bismuth plate coated with radio-tellurium (polonium) after Marckwald's method. This substance emits only [Greek: alpha] rays, and is thus especially suitable for experiments of this kind. Strutt employed the method used by him to show the charge carried by the [Greek: beta] rays (Fig. 27). He found, however, that, even in the lowest possible vacuum, the electroscope rapidly lost its charge and at the same rate whether it was charged positively or negatively. This is in agreement with the results found by the writer with radium.

In the experiments of J. J. Thomson, the electroscope was attached to a metal disc placed 3 cms. from the plate of radio-*tellurium. A very low vacuum was produced by Dewar's method by absorbing the residual gas in cocoanut charcoal immersed in liquid air. When the electroscope was charged negatively, an extremely slow rate of leak was observed, but when charged positively the leak was about 100 times greater. This showed that the polonium gave out large quantities of negative electricity, but not enough positive to be detected. By placing the apparatus in a strong magnetic field, the negative particles were prevented from reaching the electroscope and the positive leak was stopped.

These results indicate that these negative particles are not projected with sufficient velocity to move against the repulsion exerted by the electrified body, and are bent by a magnetic field. There thus seems little doubt that a stream of negative particles (electrons) is projected from the active surface at a very slow speed. Such low velocity electrons are also projected from uranium and radium. It is probable that these electrons are