556 Prof. K. T. (Jompton on Ionization and Production 



different from any hitherto proposed, and was in progress 

 when the papers of Horton and Miss Davies and of Franck 

 and Knipping were published. The results corroborate the 

 work of these investigators, and, in addition, explain the 

 apparent discrepancies in conclusions which have been 

 drawn from the various experiments on the effect at 20 volts. 

 It appears that an atom may be ionized by first absorbing a 

 quantum of energy of the resonance radiation from neigh- 

 bouring atoms, and then being struck by an election whose 

 kinetic energy might be insufficient to ionize an atom in the 

 normal state. In other words, the energy of ionization is 

 supplied partly as radiant energy and partly as energy of 

 impact. Ionization at 20 volts is therefore a secondary 

 effect, which may or may not be important, depending on 

 conditions. 



The possibility of ionization by this process is obviously 

 suggested by Bohr's theory. It appears to have been first 

 pointed out by Richardson and Bazzoni*. The proof of its- 

 existence in the present experiments is due to the use of an 

 apparatus which permits not only the differentiation of effects 

 due to ionization and to radiation, but also the estimation of 

 the proportion of the effect due to either when both are 

 present. 



Apparatus and Method. 



The ionization chamber (fig. 1) is similar in principle to 

 that used by Franck and Hertz. A tungsten filament F of 

 0'8 cm. length and 0*08 mm. diameter serves as a source 

 of electrons. These are drawn toward the platinum gauze G 

 by an accelerating difference of potential V a . Those which 

 pass through the openings in the gauze encounter a retarding 

 difference of potential v r , which is enough larger than V a to 

 prevent any of the electrons from reaching the electrode E. 

 This electrode is connected with a quadrant electrometer, and 

 it may gain a positive charge either from positive ions pro- 

 duced by ionization of the gas or as a result of electrons 

 emitted photoelectrically from E by ultra-violet radiation set 

 up by impacts of the electrons. The distance between F and 

 G is 3*5 mm., and that between G and E is 7 mm. 



In order to distinguish between these two causes of electro- 

 meter deflexions, the electrode E is constructed so that the 

 area presented to the radiation may be varied without altering 

 the geometrical relations " over all." This is done by closing 



* Nature, xcviii. p. 5 (1916). 



