182 



of 0,56 m.m. and ai^on of 0,36 m.in. pressure. Especially fig. 3 

 shows tlie efficiency of the method. In spite of the comparatively 

 small percentage of helium, the two first excitation tensions of 

 helium, though they lie above the strong excitation tensions of neon, 



Fig. 2. 



Fig. 3. 



Stand out as two sharp maxima at a distance of 0,8 Volt. These 

 maxima were used to obtain the absolute value of the excitation 

 tensions of neon, in which the value of 20,45 Volts measured by 

 Franck and Knipping ^) for the lowest excitation tension of helium, 

 was used as basis. The values thus obtained appeared to be inde- 

 pendent to a high degree of the circumstances of the experiment. 

 This method is entirely unsuitable for the measurement of the 

 ionization-potential. For then the impinging electron or the electron 

 that has been liberated from the atom by the collision can have 

 the velocity zero after an ionizing collision, also when the energy 

 of the colliding electron was greater than the work of ionization. 

 As moreover at first the effect brought about by ionizing collision 

 evidently rapidly inci'eases with increasing tension, the curve shows 

 no maximum here, but only a rise, which is besides influenced b}^ 

 the positive ions, and does not admit an accurate determination of 

 the ionization-potential. For this reason the arrangement usual 

 with a strong counter-field between N^ and P was used for the 

 measurement of the tension of ionisation for some measurements; 

 most observations were, however, made according to a new method, 



1) J. Franck and P. Knipping, I.e. 



