91 



to radiation the majority of the electrons only imparting tlieir energy 

 to tiie neon-atoms aftei- falling throngh a potential-difference eqnal 

 to the ionization-potential, thns cansing iojiization. 



In consequence one wonld expect a great diflference in the ei'ti- 

 ciency of the first non-elastic impact in neon and argon. Preliminary 

 experiments concerning the relative value of the efficiency in these 

 gases however have shown, that this difference is not large enough 

 to explain the different behaviour. So there must be another reason. 

 Beside the excitation-potential and the efficiency there is only one 

 quantity which determines the number of the non-elastic impacts, 

 and that is the mean free path of the electrons. Up to now it was 

 assumed, that the value derived from the kinetic theory of gases 

 for particles of infinitesimal small dimensions and large velocity, 

 viz. 4 l/2 times the mean free path of a gas-molecule, should hold for 

 the electrons. Recently however, H. F. Mayer ^) and C. Ramsaueh *) 

 have found, from the measurement of the mean free path of electrons, 

 that also for slow moving electrons this quantity depends on the 

 velocity of the electrons, this dependence being different for different 

 gases. Especially between neon and argon Ramsauer found a very 

 marked difference. While in neon the mean free path depends only to a 

 slight degree on the velocity of the electrons and is nearly equal to 

 the value of the kinetic theory, argon shows for very slow moving 

 electrons, below 1 volt anomalously large values of (he mean free 

 path. The mean free path then decreases and becomes a minimum 

 at approx. 12 volts, the minimum being about one third of the value 

 of the kinetic theory. This fact must be of importance for the pheno- 

 mena produced by electrons passing through a gas, especially in the 

 case of argon, where the mean free path has its minimum value 

 at a potential nearly equal to the excitation potential. 



Considering the great importance of the dependence of the mean 

 free path on the velocity, not only for the understanding of the 

 action of electrons in gases, but also for the theory of the atom, it 

 appeared desirable to me, to verify this dependence by direct ex- 

 periments, in order to obtain accurate values for the ratio of the 

 mean free paths in neon and argon, this ratio being of importance 

 for the evaluation of comparative measurements in the two gases. 

 The applied method is based on the following idea: If in an appa- 

 ratus of given geometrical dimensions electrons of a certain velocity 

 are allowed to move in a rare gas in a space, in which there is 



^) H. F. Mayer, Ann. d. Phys. 64, 451, 1921. 

 *) C. Ramsauer, Physik. Zeitschr. 22, 613, 1921. 



