chapter Seventeen 



THE MOBILITY OF CAESIUM ATOMS 

 ADSORBED ON TUNGSTEN 



A HEATED tungsten filament in presence of Cs vapor adsorbs Cs atoms. The 

 number of atoms per unit area, o, can be measured (if a<3.6 X lo^^ cm"^) 

 by suddenly heating the filament (surrounded by a negatively charged 

 cylinder) and observing the ballistic kick on a galvanometer, since every 

 atom escapes as a positive ion. 



Three cylinders placed end to end surrounded a straight filament. With 

 all three cylinders at positive potential (22 volts) a Cs film was formed 

 with the filament at low temperature. The Cs vapor was then frozen out 

 by immersing the bulb in liquid air. The filament was next heated to a 

 temperature (848° K.) at which a was reduced below 3.6 X 10^^ and the 

 temperature was then lowered to Ti at which the rate of evaporation of 

 atoms was very low. The potential of the central cylinder was then changed 

 to —22 volts leaving the other two at +22. This allowed the Cs on the 

 central part of the filament to escape as ions. The current to this central 

 cylinder gave the rate of evaporation. After the removal was nearly com- 

 plete, the potential of the central cylinder was altered to —44 volts and 

 because the accelerating field for ions then reached into the end cylinders, 

 an additional length a was freed from Cs. The current-time integral per- 

 mitted the total length of cleaned filament to be measured. 



In this way a standard Cs distribution was obtained in which the central 

 part, free from Cs, extended at each end a distance of 0.165 cm into the 

 end cyHnders, while the two end sections were uniformly coated with Cs to 

 a concentration of a = 2.73 X 10^^. 



With the filament in this standard condition the temperature was 

 changed to To for a time to while all three cylinders were at -I-22 volts so 

 that no ions could evaporate. To was chosen so low (<848°) that there 

 was no appreciable change in o by atom evaporation during the time ^2- 

 During this time, however, the Cs migrated to some extent from the end 

 sections into the central section. The number of atoms which had arrived 

 were then measured by flashing the filament while the three cylinders were 

 put at — 22 volts, but the current to the central cylinder only was measured. 



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