102 



Prof. A. LI. Hughes on the Velocities of 



of a research on the selective effect which would seem to 

 show that the electrons emitted in the selective effect were 

 on the whole somewhat slower than those emitted in the 

 normal effect. 



It was therefore thought desirable to investigate the 

 velocities of the electrons emitted in the selective and normal 

 effects. As the evidence available tends to show that the 

 distribution of velocities of the electrons is the same in both 

 cases, the apparatus was designed to show whether this was 

 true or not. To find out exactly the nature of the difference 

 (if any) in the distribution of velocities would require a more 

 complicated apparatus. It is highly desirable when working 

 with sodium-potassium alloy, which is so sensitive to traces 

 of reacting gases, to keep the apparatus as simple as possible. 



The apparatus consisted of a bent glass tube about 3 cm. 

 wide (fig. 2). The inside was plated with a semi-transparent 



^-^ I mercury larrp. 



Fig. 2. 



i variabfe aoirature: 



\\\\---\^r-.eartk 



film of platinum except for the rounded ends which acted as 

 windows. A guard-ring Gr served to prevent any electri- 

 fication from leaking over the glass from the platinized 

 surface to the electrode E. Sealing-wax around the outside 

 of the platinum seal making connexion with the electrode E 

 gave satisfactory insulation. The sodium-potassium alloy 

 was strongly heated before being put into the apparatus to 

 drive off occluded gases. It was then placed in the first 

 bulb connected with the apparatus (fig. 3), which was 

 exhausted by means of a Gaede mercury-pump and charcoal 

 cooled by liquid air. The apparatus was strongly heated for 

 several hours while the exhaustion proceeded, and then, by 

 suitably tilting the apparatus, the alloy was filtered from the 

 first bulb into the second. The alloy was heated for several 

 hours again and then transferred to the third bulb. The 

 apparatus was now sealed off from the pump and allowed to 



