286 PHENOMENA, ATOMS, AND MOLECULES 



a point source of this radiation, the rate of production of metastable atoms 

 should vary approximately inversely as the cube of the distance from the 

 end of the arc. The observed values are in accord with this theory. 



The electrons in the tube generated at the walls by the metastable atoms 

 give to the neon a conductivity whose value can be measured, and from the 

 known mobility of the electrons in neon gas, the concentration of electrons 

 can be calculated. For low voltages on the collectors, the currents are de- 

 termined solely by this mobility of the electrons, and the volt ampere 

 characteristics for small currents are thus linear on both sides of the zero 

 point. For larger negative voltages, the current becomes limited by the 

 number of electrons that can escape from the collector. For sufficient 

 positive voltages, the currents increase more rapidly than corresponds to the 

 linear relationships, this being due to ions produced within the tube by the 

 acceleration of the electrons. The experiments prove that these ions are 

 produced by the ionization of metastable atoms rather than normal atoms, 

 for the rate of production of ions varies approximately with the square of 

 the light intensity. These conclusions are confirmed by experiments with 

 a shutter placed beyond the end of the arc, which can cut off the radiation 

 without interfering with the diffusion of atoms, ions, etc., past the shutter. 

 It is thus proved that the production of metastable atoms is produced by 

 light which travels only in straight lines. The fact that when the collector is 

 positive the electron currents are also proportional to the light intensity 

 proves that the electrons owe their origin not to photoelectric effect from 

 the electrodes, but to the production of metastable atoms or excited atoms. 

 Small disk-shaped collectors have also been used in which one side, which 

 faces the source of resonance radiation, is covered with mica so that the 

 exposed surface cannot receive radiation, and yet the electron current was 

 nearly as great as if the mica were placed on the back side of the collector, 

 proving that the electron currents are generated by metastable atoms which 

 diffuse short distances to the electrode even when the resonance radiation 

 does not reach the electrode itself. 



Since the electrons emitted by the collectors possess considerable initial 

 velocities, and since these electrons make elastic collisions with the neon 

 atoms, it is impossible to obtain a saturation current from these electrodes 

 at these gas pressures, for electrons which diffuse back to the electrodes 

 have sufficient energy to be reabsorbed by it. A theoretical investigation 

 shows that in this case the current 4 that flows between any two electrodes 

 is given by the equation, 



ix=— /oXC(F/7o)- ^ 



logCl + F/Fo) 

 where V is the potential difference between the electrodes, Vo is the volt 



