SOME CONTEMPORARY ADVANCES IN PHYSICS 



109 



gradually overcomes the space-charge repulsion, augmented in the 

 gas by the reflection of electrons, for the reflected electrons stay 

 longer in the space between filament and plate than they would if 

 they went straight through. In argon the curve rises at first more 

 swiftly, almost or quite as steeply as in vacuo, for the atoms are 

 almost transparent to the electrons when they are slow; but as their 

 speed is increased and the effective radius of the atom rises, the cur- 

 rent sharply declines again. Further on, near 11 volts, there is 



JO 



\Z5 



>75 



5 10 15 20 



Besch/eunmende Spannung in Voft — 



Fig. 8 



Z5 



another peak; near this voltage, the electrons which collide with 

 atoms lose almost all their energy (threshold-speed for inelastic im- 

 pact at 11.3 equivalent volts) at the first collision, and pass through 

 the rest of the gas-filled region without obstacle. A second peak 

 near 16 volts is ascribed to a second critical speed for inelastic impacts. 

 Krypton and xenon give toothed curves of the same general type. 

 Neon and mercury vapor, however, behave like helium, the curves 

 rising steadily or at most showing slight kinks and inflections which 

 may be indications of a slight effect of the same sort. 



The reason for this remarkable effect is still obscure. It may be 

 possible to devise an atom-model adequate to explain it without 

 forfeiting spherical symmetry, which it is desirable to retain if pos- 

 sible, for among all atoms these of the heavy inert gases would be 



