a Particles ivith Light Atoms. 



565 



fig. 2. The a rays from a slanting source R passed through 

 a film of paraffin wax about 30 //, thick placed at the end of 

 two parallel insulated brass plates A and B, 6'02 cm. long, 

 and 0*155 cm. apart. These were mounted on an ebonite 



Fit 



iri 



To pump. 



frame DD with circular ebonite ends which slipped into a 

 glass tube T. A brass plate with a slit 1 cm. long and 3 mm. 

 wide covered with a thin silver plate of stopping power 6 cm. 

 of air, was fixed at the end of the tube T. The zinc sulphide 

 screen was mounted outside close to the silver plate. The 

 electric connexions with the plates A and B were made 

 through ground-glass stoppers shown in the figure. 



As radium C was employed as a source, it was necessary 

 to arrange for rapid exhaustion of the apparatus to stand 

 5000 volts between the plates after a short interval. After 

 preliminary evacuation by a Fleuss and Graede mercury-pump 

 a Langmuir pump was used, and the process was so rapid that 

 the necessary vacuum was reached and lield within two 

 minutes of introducing the apparatus into the glass tube. 



In order to deflect completely the H atoms in passing 

 between the parallel plates, it was calculated that about 

 30,000 volts would be required. Apart from the difficulty 

 of obtaining rapidly a vacuum sufficient to support and 

 maintain such a voltage, a steady supply of not more than 

 7000 volts was available in the Laboratory. To overcome 

 this difficulty, it was arranged to compare the deflexions of 



