564 Prof. K. T. Compton on Ionization and Production 



inconceivable that other atoms at the large relative distances 

 involved in these pressures could change the entire nature 

 of the effect of an electron impact. The observed ionization 

 must therefore be a secondary effect due. at least in part, to 

 the radiation, or it must be due to electron impacts against 

 atoms in such rapid succession that the energies of the 

 impacts are additive in their effect. The latter possibility 

 may be discarded, owing to the extremely long time during 

 which an atom would have to retain energy from an impact 

 in order to account for the observed results at these low 

 pressures and current densities*. No secondary effect of 

 the radiation which has been suggested, such as ionization 

 by electrons photoelectrically emitted from the filament or 

 the electrode E, seems at all adequate to account for the 

 ionization and the variation of the proportion of ionization 

 <md radiation with pressure, except that which ascribes the 

 ionization to electron impacts against atoms which, at the 

 instant, are in a relatively unstable condition because they 

 have absorbed radiant energy coming- from neighbouring 

 .atoms which have been previously struck. In fact, some 

 such phenomenon must occur, since the energy radiated is 

 strongly absorbed and re-emitted as resonance radiation, and 

 is thus passed on from atom to atom. 



At the higher pressure it is obvious that no appreciable 

 ionization can occur as a result of single impacts, since the 

 electrons, however and wherever emitted, would collide so 

 frequently while gaining the energy between 20*2 and 

 25*5 volts that they would certainly collide inelastically 

 and produce 20*2-volt radiation before acquiring the 25'5- 

 volt energy necessary for ionization. Thus practically all 

 ionization observed at higher pressures, whatever be the 

 applied voltage (unless this is so large as to give a potential 

 drop of the order of magnitude of a volt in a mean free 

 path), must be due to a secondary effect involving combined 

 action of radiant energy and direct impact. 



This point is well illustrated by fig. 5, for which the 

 pressure was 8 mm. Here neither curve shows any indi- 

 cation of a break at 25 5 volts, while the R-V a curve 

 shows that the effect from 20'2 volts up is very largely due 

 to ionization. Thus this entire curve, in which the actual 

 ionization currents were very large indeed, seems due to 

 20*2 volt impacts. The shapes of these curves differ from 

 those for lower pressures. The maximum and succeeding 

 decrease in the E-V a curve is due to the fact that, as 

 V a increases, more and more of the ionization occurs on the 

 * K. T. Compton, Phys. Rev. (in print). 



