340 BELL SYSTEM TECHNICAL JOURNAL 



It was not found practicable, even with such exact control of the 

 silver oxide film, the amount of caesium and of the temperatures of 

 the photo-cell parts, to give the cell a definite time-temperature cycle 

 in the transfer process. It is necessary to follow the growth of the 

 photo and thermionic currents as the heating continues. A small 

 tungsten lamp is mounted in a fixed position with respect to the cell 

 and after the heating has progressed for some 20 minutes, observations 

 are made each minute on the total thermionic and photoelectric 

 currents. These currents rise in value as the surface is built up and, 

 by experience, a definite point on the growth curve is found where 

 the hot air stream should be discontinued in order to obtain the 

 optimum surface. 



After the cell has cooled it is sealed from the exhaust system if a 

 vacuum cell is desired, or filled to the required pressure with an inert 

 gas, usually argon, if a gass filled cell is desired. 



Surface Structure 



In the study of the photosensitive cathode surface some information 

 regarding the physical structure and chemical nature of the surface 

 has been gained by direct examination and analysis. The physical 

 nature of the roughened silver may be seen from microphotographs 

 of the cathode taken following the roughening oxidation and reduction 

 which precedes the quantitative oxidation. 



Fig. 2a is a plan view taken at 1,530 diameters, magnification. 

 Fig. 2h is a transverse section of the cathode supported in a heavy 

 nickel plate taken at 200 diameters. Fig. 2c is a detail view of the 

 front surface of the transverse section taken at 2,450 diameters. 

 It would appear that the effect of the oxidation and reduction is to 

 etch out the polished silver sheet, giving the surface elements a random 

 orientation but not causing any great increase in surface area. If we 

 judge the length of a line element to be doubled, this indicates a 

 four-fold increase in area. 



To determine the amount of oxygen entering into combination at 

 each discharge of the condenser, three cells were given 50, 75 and 

 100 taps respectively of the key controlling the condenser discharge. 

 The excess oxygen was removed and the exhaust manifold trapped off 

 by a mercury seal. Each cathode in turn was then heated by induced 

 high-frequency current till the silver oxide decomposed, and the 

 pressure developed in the exhaust system was measured. The 

 pressures and amounts of oxygen as microgram-molecules of O2 are 

 given in Table 1. 



