328 



PHENOMENA, ATOMS, AND MOLECULES 



part of either filament could be measured. The tube is highly evacuated 

 but contains saturated caesium vapor whose pressure is controlled by the 

 tube temperature. The rate of arrival of atoms, \ia, is measured with B 

 at a temperature of ^-^ 1200° K. This steady current is then balanced out 

 so that only changes in current are indicated by the galvanometer (G) 

 connected to co ; Ci, Co and f were usually maintained at 45 volts negative 



-©- 



inr 



Fig. 2. Filament and collector arrangement in two filament method (diagrammatic). 



to filament B. The potential of filament A is made 41 ± 2 volts negative to 

 5 (4 ± 2 volts positive to Ci, C2 and Co)- Filament A is then cleaned by 

 flashing at 2400° K. After a steady state (6) is reached on its surface at 

 any of a series of lower temperatures (300 — 1200° K.), the temperature 

 of A is suddenly raised to about 1800° so that at these voltages all the 

 adsorbed atoms evaporate instantly as neutral atoms, which travel in 

 straight lines towards the cylinders c. A definite fraction / of these, how- 

 ever, are intercepted by the hot filament B and are converted into ions 

 which pass to the cylinder Co- The resulting ballistic kick indicated by the 

 galvanometer, together with the known diameters of A and B and the 

 distance of A from B give data for the calculation of Oa and 6. 



The use of the second filament 5 as a means of measuring concentra- 

 tions on filament A too high to be flashed directly from A as ions was 

 suggested by Dr. Tonks of this laboratory. 



The potential chosen for filament A insures that neither ions nor 

 electrons shall pass to Co as the adsorbed atoms are suddenly evaporated. 



