IV. F. Holinan 



EXPERIMENTS 



Zinc, carefully polished and cleaned, was tested first. This 

 was mounted in the electrode chamber, which was immediately 

 evacuated and left at an indefinitely low pressure in connection 

 with the drying chamber for several hours. 



With a gas pressure of .ooi mm., a P. D. of about 500 volts,^ 

 and a capacity, C, of about 1,400 cm., the photo-electric current 

 produced in ten seconds the following series of deflections of the 

 electrometer (8.5 scale divisions=one volt). The arc was 

 stopped after each reading and started anew for the next. 



Table I 



33.4 

 34.4 

 32.9 



27.0 



This value is equivalent to 6X io~^" amperes. After admitting 

 hydrogen at a pressure of 1.5 mm., a glow current was then sent 

 from the zinc as cathode for about fifteen minutes, and the cham- 

 ber again evacuated to a pressure less than .001 mm.. The photo- 

 electric current observed at this point gave the following suc- 

 cessive values : 



Table TI ■ "^ 



20.2 20.2 20.4 



20.5 16.1 19.8 



20.2 19.5 20.6 



20.2 20.6 21.1 - 



Mean 20.0 



representing a current of 3.8X10""'" amperes, which is a marked 

 decrease compared with the fresh metal. 



'At the pressures used tlie photo-electric current proved in all cases 

 wholly independent of the magnitude of the P. D. between the electrodes. 



130 



