ABSTRACTS OF RECENT TECHNICAL BOOKS 657 



The positixc ion emission is constant while the temperature de- 

 creases from a high \alue to a low critical temperature. Here the 

 ion emission drops suddenly while some caesium sticks to the filament. 

 Further decreases in temperature are followed by increased adsorp- 

 tion and decreased ion emission. If the temperature is then increased 

 in steps the ion current retraces its path. At an upper critical tem- 

 perature, about 50° higher than the lower critical temperature, the 

 filament cleans itself spontaneously, the caesium comes off as ions 

 and registers as a sudden rush of current. At higher temperatures the 

 ion current has its initial constant value which is limited by the arrival 

 rate of caesium atoms. The critical temperatures are raised by an 

 increase in the vapor pressure or l)y a decrease in the plate 

 potential. 



A method of determining the amount of adsorbed caesium is de- 

 veloped. At a sufftciently high filament temperature the surface is 

 clean. At a sufficiently low temperature every atom that strikes the 

 filament sticks to it, at least until the optimum activity is reached. 

 The product of the arrival rate, which is given by the steady positive 

 ion current, and the time to attain the optimum activity gives the 

 number of caesium atoms at the optimum activity. At an inter- 

 mediate temperature the surface is only partly covered. If the 

 temperature is suddenly dropped, to a low value, it takes a shorter 

 time to reach the optimum activity. From these times the amount 

 of adsorbed caesium at various temperatures, plate potentials, and 

 vapor pressures can be determined. At the optimum activity there 

 are 3.7 XlO'"* atoms of caesium on a cm- of tungsten. This is very 

 nearly the same as the number of caesium atoms that could be packed 

 in a single layer, but is considerably smaller than the number of 

 caesium ions in such a layer. 



The adsorption or evaporation characteristics are illustrated by 

 curves. Caesium can evaporate either as ions or as atoms. The 

 atomic rate depends only on the temperature and on G, the fraction 

 of the surface covered with caesium. For a given temperature it 

 increases very rapidly with 9.. The ions can permanently escape 

 from the filament only if the potential is in the right direction. A 

 typical isothermal ion rate curve increases rapidly with 6, comes to a 

 maximum when is about .01, then decreases continuously for larger 

 6. These curves explain all the observed phenomena of these adsorbed 

 films. They show that while the ion work function increases as 

 increases, the work to remove an atom decreases with 0. The ion 

 work function for a given can be decreased by increasing the po- 

 tential gradient at the filament. 



