566 Dr. C. B. Bazzoni : Experimental Determination 



a rectangular hyperbola. Beyond this critical bend, the 

 curve approaches more or less rapidly to a straight line 

 parallel to the axis o£ abscissa?. The above, in connexion 

 with previous results on the collapsing pressures of long 

 tubes, thus should allow us to calculate with reasonable 

 approximation the collapsing pressures of short steel tubes 

 of these thicknesses. 



Laboratory of Physics, University of Illinois, 

 Urbana, Illinois, May 1916. 



LXII. Experimental Determination of the Ionization Potential 

 of Helium. By Chakles B. Bazzoni, Ph.D., Harrison 

 Research Fellow, University of Pennsylvania*. 



rg^HE energy of impact necessary to remove an electron 

 JL from simple atoms can be calculated directly from 

 Bohr's theory. The application of Bohr's formula to helium 

 indicates that an energy corresponding to that of an electron 

 falling through 29*3 volts is necessary to remove one elec- 

 tron from the helium atom, and that an energy corresponding 

 to a fall through 83'4 volts is necessary to remove both 

 electrons. If the theory is correct, no assumed configuration 

 of the electrons in the atom can give any other probable 

 values but these, provided that impact ionization, so called, 

 results directly and solely from impacts as is generally 

 assumed. It follows that an experimental determination of 

 the ionization potential of helium ought to serve as an 

 excellent direct check on the validity of Bohr's theory itself. 

 The first determination of this potential was made by 

 Franck and Hertz t using a very sensitive method for 

 detecting small amounts of impact ionization. The value 

 obtained w T as 20*5 volts. The only other determination has 

 been made by Pawlow J, who used a method essentially the 

 same as Franck and Hertz, and got a result, 20 volts, in 

 excellent agreement with those investigators. Pawlow, 

 however, found it unnecessary to apply any correction for 

 the initial velocity of the impinging electrons, while Franck 

 and Hertz's result involves such a correction. Recently 

 K. Compton § has calculated the ionization potential, making 

 use of a formula w T hich he has developed and of various 

 recorded data on helium ionization. He obtains 22' 5 volts 



* Communicated by Prot. O. W. Richardson, F.E.S. 

 t Ber. der B. Phijs. Ges. 1913, p. 34. 

 % Proe. Roy. Soc. vol. xc. p. 338 (1914). 

 § Phys. Rev. 2nd ser. vol. vii. pp. 4 & 5. 



