EVAPORATION OF ATOMS 



349 



ever, the value 2.8 volts, obtained by Langmuir and Kingdon,^^ for a 

 heavily Cs coated filament, by direct measurement at ^^ 300° K., suggests 

 there is no large dependence on temperature. 



It is to be noted that the maximum Vc or Vg occurs zt 6 = 0.67 and de- 

 creases as 6 approaches i.o. As ^'s of 0.5 to 0.6 are approached, the ad- 



T DEGRESS K 



Fig. 15. Field-free electron emission (calculated) from a tungsten filament in 

 equilibrium with Cs vapor at the filament temperature T. Rate of arrival (m-o) of Cs 

 and bath temperatures given on each curve. Pressure at 237° K is 6.7 X lO"' baryes ; at 

 4i2°K it is 8.8 baryes. Diagonal straight lines intersecting the curves give correspond- 

 ing values of ^n. 



sorbed Cs atoms begin to form a fairly continuous layer. The outer surface 

 of the layer begins to have the properties of Cs and not those of a com- 

 '^I. Langmuir and K. H. Kingdon, Phys. Rev., 34, 129 (1929). 



