PAPERS ON CHEMISTRY AND PHYSICS 279 



PHOTOELECTRIC EFFECT OF CAESIUM VAPOR 



AND A NEW DETERMINATION OF h, THE 



UNIVERSAL CONSTANT OF PLANCK 



Jakob Kunz and E. H. Williams, University of Illinois 



The ionization potential Vi is related to the conver- 

 gence frequency ni of the principal series in the spec- 

 trum of the gas by the expression 



Vie=hiii 

 where h is Planck's universal constant and e the charge 

 of the electron. 



In view of the results in the X-ray region, we may ex- 

 pect an interchangeability in the effect of electron colli- 

 sion and radiation, i. e., we may expect a gas to be ionized 

 whether it is struck by an electron moving with the criti- 

 cal velocity corresponding to Vi or illuminated by radia- 

 tion of the corresponding wave length. 



To test this, caesium vapor was illuminated by ultra- 

 violet light. The result was that for light below a cer- 

 tain wave length, about 318mm, ionization was produced 

 whereas above this value no effect could be detected. Sub- 

 stituting in the above equation the frequency correspond- 

 ing to the wave length 318mm and the known values of Vi 

 and e we obtain for h the value of 6.58.10 -27 ergs while 

 the accepted value determined by other methods is 

 6.55.10- 27 ergs. 



