1091 



direction due to the augmented increase of the intensity of the 

 current and which occur at regular distances '). For heliunti 



350 



JOO 



250 



200 



ISO 



100 



so 



1 



A 

 of 



20 40 60 Ö0 lUO \Zü 140 IGO IftU 20Ü Zlü VULT 



Fig. 2. 



Fhanck and Hertz have observed a similar phenomenon and proved, 

 that these changes of direction must occur each time an electron has 

 travelled through a potential ditFerence equal to the ionisation potential. 

 Our measurements give in this way for the ionisation potential of 

 argon 12,0 Volt, which agi'ees satisfactorily with the value Fhanck 

 and Hertz found in another waj'. ') In the curve for constant tield 

 the steps are much more pronounced than in that for constant plate 



1) Maximally we have observed 15 steps. 



2) Later measurements by Dr. Oosterhuis and one ol' the writers proved that 

 no real ionisation occurs at 12 Volts. The real ionising potential is about 17 

 Volts. So 12 Volts must be considered to be the velocity of first inelastic 

 impact (Note added in translation). 



