336 BELL SYSTEM TECHNICAL JOURNAL 



The cells thus obtained are highly sensitive to red and infra-red light. 

 The maximum response is at 8,000 A., the response is one third as great 

 at 10,000 A., and the photoelectric threshold is somewhere in the neigh- 

 borhood of 12,000 A. Cells are frequently obtained with a vacuum 

 sensitivity of 35 microamperes per lumen to a light at 2,710° K. color 

 temperature, and their useful life is indefinitely long, even at a temper- 

 ature of 50° C. But such results obtain only if the cells be produced 

 under controlled and definitely specified conditions. It appears that 

 the product is definitely affected by variations in the quantities of 

 cffisium and oxygen. Also, the course of the chemical reactions (still 

 obscure) and the thickness of the final caesium thin film are very 

 sensitive to slight variations in the process conditions. In the early 

 stages of this work the results were highly erratic; only occasionally 

 was a useful cell obtained. A large amount of development time has 

 been devoted in these laboratories to the isolation and correlation of 

 the various factors which determine the sensitivity of a finished cell. 

 With the technique now available, c^sium-oxygen-silver photoelectric 

 cells are prepared under conditions of quantity production with 

 sensitivities varying within a factor of two and with a process shrinkage 

 no greater than obtained in the production of high quality thermionic 

 vacuum tubes. 



The Structural Details and Method of Preparation 

 OF the Cells 



The structure of the cells used in this study is shown in Fig. 1. 

 The cathode is a semicylinder of silver 99.9 per cent pure. The anode 

 is a nickel wire mounted in the axis of the cylinder. These are 

 mounted on a stem which is sealed into a spherical bulb of soda-lime 

 glass. Around the stem is suspended an open ring of heavy copper 

 wire between the ends of which is crimped a tube rolled from thin 

 sheet molybdenum which carries the caesium pellet. This structure is 

 adapted to the initiation of the chemical reaction by induced high- 

 frequency currents with a minimum heating of the oxidized cathode. 

 A nickel shield is placed between the pellet sheath and the cathode 

 to protect the latter from radiation and to deflect the hot caesium 

 vapor evolved by the pellet. 



The essential steps in the process of formation of the active cathode 

 surface are: 



1. Formation of a silver oxide film on surface of cathode. 



2. Preparation of caesium. 



3. Combination of caesium with silver oxide surface. 



