492 Prof. McLennan and Mr. Found on Delta Radiation 



Hallwachs * have shown, too, that the removal of occluded 

 gases from potassium by repeated distillation in a very high 

 vacuum caused its photo-electric effect to disappear com- 

 pletely with light which included wave-lengths down to 

 \ = 3400A.U. The results o£ Kiistner and Wiedmann and 

 Hallwachs have also been confirmed by Fredenhagen ~\. 



In addition, Hughes J has shown that the contact 

 difference of potential between zinc or bismuth, both distilled 

 in vacuo, and platinum is exceedingly small when the surfaces 

 of the zinc or bismuth consist of fresh deposit of the distilled 

 metals. If traces of air, however, be admitted into the 

 evacuated chamber containing the metals a great increase 

 takes place in the contact difference of potential between 

 the metals. 



In view of all these experiments it was thought well to 

 investigate what the effect would be on the intensity of the 

 delta radiation from zinc under bombardment by alpha rays 

 when care was taken to remove as far as possible all gases 

 from the surface of the zinc bombarded. The following 

 paper contains an account of this investigation, and from 

 what follows it will be seen that with freshly prepared zinc 

 surfaces the delta-ray effect is exceedingly small, but that 

 when air is permitted to be occluded in such surfaces a very 

 great increase takes place in the magnitude of the effect. 



II. Apparatus, 



The apparatus used in conducting the experiments is 

 similar to that used by Hughes § in his investigations on 

 the photo-electric effect and is shown in fig. 1. It consisted 

 of a glass tube about 3 cm. in diameter and about 60 cm. in 

 length. This tube carried at its upper end a tap windlass 

 W and at its lower end it was provided with a ground-joint 

 for fitting it into the glass heating-chamber shown in the 

 diagram. The tube was lined with a thin-walled brass tube 

 which was kept joined to earth through a connexion at E. 

 B and D were two guiding-rods of brass attached to the 

 inner lining brass tube, and MN was a strip of brass which 

 was supported by a cord from the windlass W and had loops 

 on its ends about the guiding-rods B and D. An insulated 

 brass rod H was rigidly attached to MN through the inter- 

 mediary of a short cylinder of amber A. It carried at its 



* Wiedmann and Hallwachs, Verh. d. Deutsch. Phys. Ges. p. 107 

 (1914). 



f Fredenhagen, Verh. d. Deutsch. Phys. Ges. p. 201 (1914). 



X Hughes, Phil. Mag., Sept. 1914, p. *337. 



§ Hughes, Phil. Trans. A. ccxii. p. 205 (1912). 



