330 



PHENOMENA, ATOMS, AND MOLECULES 



temperature. For example, a 2 mil filament requires,-^ 15 seconds to cool 

 from 1220° K. to 600° K. A filament in a bulb containing caesium at room 

 temperature is completely coated in less than one second. The method also 

 fails at high pressures because the ^'s to be measured may already be above 

 the optimum. 



We have often found another serious difficulty in the application of 

 Becker's method. Fig. 3 shows emission vs. time curves taken as in Becker's 

 method. The curve a with a single maximum is obtained for caesium on a 

 clean homogeneous tungsten surface. With extremely minute traces of 

 gaesous impurities which are sometimes hard to avoid but which do not 

 interfere with the application of the other methods of determining $, we 

 have often obtained two maxima as shown in curve b. With slight modifica- 

 tion of the conditions the two maxima may merge into one. In such cases 

 difficulties of interpreting the curves may lead to considerable error in the 

 determination of 6. 



15 



-\ 10 



K 



O 

 JO 



200 240 aeO 3ZO 360 /JOO 440 ^too 



55 



70 75 60 



t (seconds) 



Fig. 3. Electron emission vs. time curves as obtained in Becker's method for measuring 

 6. (a) Clean filament, (b) partly gas covered. 



(b) The direct flashing method is applied only below 6 ^ 0.08 but is 

 so extremely sensitive that 6 as low as 10"^ can be measured easily. 



(c) The accumulation method may be used to produce any value of 6 

 provided |i„ is kept below ,_/ 10^^ to give sufficient time for observations. 

 It should find a useful application in the study of contact potentials. Known 

 changes in the surface condition of an electrode receiving electrons can be 

 produced. The resultant changes in contact potential could then be measured 



