183 



ill wliicli a very reliable criterion is applied for tlie first appearance 

 of the ionization. For tiiis purpose a second, very thin incandescent 

 wire G of the shape represented in Fig. 5 was placed in (he field- 



c.ir. 

 S 





? 



.0 



Fig. 5. 



free space R; the positive end of this wire (on the left side of the 

 figure) was connected with the walls of R, so that the field round 

 the wire, which moreover remains lestricted to its immediate neigh- 

 bourhood on account of tlie slight thickness of the wire, can by no 

 means accelerate the electrons coming from D. This incandescent 

 wire was heated to such a temperature that the stream of electrons 

 flowing from the wire to the metal wall, is limited by the space 

 charge. So long as the energy of the electrons coming from D is 

 not sufficient for the formation of positive ions, they have no in- 

 fluence at all on the amount of the stream of electrons issuing from 

 G. Nor could a photo-electric effect, if it took place, even apart 

 from the fact that it is so small, have any influence on the amount 

 of this stream of electrons, limited by the space charge. As soon, 

 however, as positive ions are formed, and some of them get into 

 the neighbourhood of G, the space charge is partly annihilated, and 

 the stream starting from G suddenly lises. Figures 6 and 7 show 

 the results of these measurements in neon and argon. It is seen that 

 not even the slightest discontinuity can be [)erceived in the curve 

 when the lower excitation tensions are passed, while at the tension 

 of ionisation the stream begins to rise rapidly. To obtain the absolute 

 value of the ionization potential, the maximum corresponding to the 

 first excitation potential was determined at the same time by the aid 



12 

 Proceedings Royal Acad. Amsterdam. Vol. XXV. 



