CMSIUM-OXYGEN-SILVER PHOTOELECTRIC CELL 339 



great rise in temperature and causes the immediate and complete 

 expulsion of all caesium. This allows the high-frequency heating 

 with the attendant hazard of heating the cathode and reducing a 

 part of the silver oxide to be a minimum. The caesium travels in 

 straight lines from the pellet housing and is condensed on the glass 

 wall of the bulb. A shield above the housing prevents any caesium, 

 as it is expelled from the pellet, from impinging on the cathode surface. 



3. Transfer of CcEsium to the Cathode Surface 

 The transfer of the caesium to the cathode surface and its reaction 

 with the silver oxide film to the proper extent is probably the most 

 difficult process to control. The ideal process requires the transfer 

 of the caesium from the glass wall of the envelope to the cathode 

 surface without the reduction of any of the silver oxide by the heat 

 required to bring about this caesium transfer, the reaction of the silver 

 oxide with the greater portion of the caesium and the leaving of a 

 sufficient amount of uncombined caesium to supply the required 

 volume concentration of caesium and to cover the entire cathode 

 surface with an equilibrium thin film of caesium. The practical 

 difficulties in carrying out this process are due to the fact that, at the 

 temperatures required to transfer in a reasonably short time the 

 caesium from the glass wall to the cathode, the silver oxide has an 

 appreciable rate of decomposition. This difficulty was overcome so 

 far as is possible by heating the glass wall selectively at a controlled 

 temperature. There is a material lag in the cathode temperature in 

 this process. Thus the cathode is kept at as low a temperature as 

 possible while the caesium is transferred to it at a sufficiently rapid rate. 

 This is accomplished by surrounding the cell with a stream of hot 

 air for approximately 30 minutes. A glass chimney is placed around 

 the cell. The chimney fits into a transite manifold containing heating 

 coils through which compressed air is fed at the rate of 0.5 liter per 

 second per chimney. The temperature of the air stream is controlled 

 to within 5° C. 



If this process is carried on in the usual electric oven where a portion 

 of the winding is exposed, the cathode is heated preferentially due to 

 the radiant heat and it is almost impossible to obtain an active surface 

 due to silver oxide reduction by temperature, as well as to the fact 

 that with the cathode the hottest surface, the caesium will tend to 

 condense elsewhere. Even with all windings of the oven covered by 

 asbestos sheet so that substantially all heat is due to convection, the 

 process is infinitely more difficult of control than with the hot air 

 stream oven. 



