'06 



Radhun Radiation and Contact Electricity, 



two metals under the influence o£ radium radiation when 

 they were placed in an exhausted vessel. 



Measurements of contact difference of potential in high 

 vacua have been made by Dr. J. T. Bottomley "^ and also by 

 Mr. F. S. Spiers t- Fig. 3. 



The former measured the potential- 

 difference between zinc and copper at 

 a pressure of less than 1/523 mm. of 

 mercury, and found that w^hen air w^as 

 admitted to atmospheric pressure the 

 potential-difference did not alter by so 

 much as 1 per cent. Mr. Spiers, after a 

 series of careful experiments, in w^hich 

 the pressure w^as reduced to 1/10000 

 mm. of mercury, came to the conclusion 

 that no mere mechanical pumping is 

 capable of removing '' the insoaked 

 condensed air-films from the surfaces 

 of such highly oxidizable metals as 

 zinc and aluminium,^'' to the presence of 

 which he attributes the volta-effect % • 



In view of the results obtained by 

 these experimenters it is scarcely sur- 

 prising that the potential -difference 

 under the influence of radium radiation 

 show^ed no variation as great as 5 

 per cent, when air to atmospheric pres- 

 sure w^as admitted to a vessel pre- 

 viously exhausted to 1/7000 mm. of 

 njercury. 



The experiment was carried out with 

 a " vacuum tube/^ designed for the 

 purpose, and constructed by Casella. 

 One electrode was of platinum, the 

 other of aluminium. It was found that 

 the heat required to seal the aluminium 

 electrode into the tube was sufficient 

 to oxidize the surface and completely 

 destroy the volta effect. The tube was 

 therefore made as shown in fig. 3, 

 with the aluminium electrode attached to a ground-in stopper, 

 so that it could be removed and cleaned before use. The joint 



* B. A. Report, 1885. 

 t Pliil. Mag. vol. xlix. p. 40 (1900). 



X The volta-effect would be diminished by " insoaked condensed air- 

 films '■ if present on zinc or aluminium &urfaces.-^EELAa>r. 



