12 Photo-electric Discharge from Metallic Surfaces. [Apr. 24, 



Series of curves were obtained, showing the relation between the 

 photo-electric current and the potential at pressures ranging froi 

 760 mm. to 0-0014 mm. They show that at pressures above about 

 1 mm.-the actual pressure depends on the distance between the 

 *lectrodes-the current increases at first rapidly with the potential 

 then less rapidly, and finally, when a certain critical potential gradient 

 has been reached, more rapidly again. No true saturation currents 

 were obtained at these pressures, though the middle portion of 

 curves were always less steep than the other portions. The currents 

 for the less steep part of the curves increased some twenty! old in value 

 as the pressure was reduced from that of the atmosphere down to a 

 pressure of about 1 mm. Below this pressure the current again 

 decreased, and 'soon perfect saturation currents were obtained, which 

 became smaller as the pressure was further diminished, though 

 approaching a finite limit. 



Curves connecting the potentials and corresponding photo-electric 

 currents in air, carbon dioxide, and hydrogen at various pressures, 

 were also obtained. They show that at the higher pressures the 

 currents in carbon dioxide are about 1-75 times those in hydrogen, 

 and 1-3 times those in air, for corresponding points on the less steep 

 portions of the current potential curves. For the upper part of the 

 ourve, when we have ionisation by collision becoming the predominant 

 factor, the relation between the currents is quite altered. 



The curves obtained could, however, all be explained on the ionic 

 theory of conduction both qualitatively and quantitatively. 



Experiments were also carried out, and the corresponding curves 

 drawn, for the photo-electric currents in carbon monoxide, and for the 

 currents using other electrodes than zinc ; the results obtained are 

 .given at length in the paper. 



