300 Prof. Richardson and Lieut. Bazzoni on the 



value for 29*3 volt impacts as determined from the eY = hv 

 relation. The formula gives a frequency of 7 # 12xl0~ 35 , 

 and a wave-length of 422xl0~ 8 cm. Our experimental 

 results show that the limiting wave-length lies between 

 X 425 and A, 462 (the value for 1*9 ampere), but the trace 

 of an effect at 2 amperes makes it probable that there is 

 radiation close to \ 425 ; and since the shape of the distribu- 

 tion curve (fig. 3) is such that no appreciable effect is likely 

 to come from values corresponding to the extreme edges of 

 the slit, no marked effect at 2 amperes could be expected 

 from A, 422 if it were present. This is an indication that 

 the ionization potential of helium is 29*3 volts as given by 

 Bohr's calculations, but the evidence of most other experi- 

 ments * strongly indicates that the potential is actually 

 20 volts. If the maximum energy corresponded to 20 volt 

 impacts, a radiation of frequency about 5 x 10 15 would be 

 liberated, and the spectrum in these curves ought to ter- 

 minate at about 1*6 amperes. This suggests again the 

 probability that we have put forward heretofore f, that gas 

 ionizations are not simple impact phenomena but relatively 

 complex processes in which radiation plays a part. It may 

 be that a 30 volt impact is necessary to ionize a normal 

 helium atom, but that 20 volt impacts only are required on 

 atoms thrown into an abnormal state through the absorption 

 of 20 volt radiation. This is a point which we have not been 

 able to settle to our complete satisfaction. Whatever may 

 be the theoretical significance of the observation, the fact 

 remains that this radiation is shorter than has ever hereto- 

 fore been located in this region. 



Concerning the distribution of energy in this spectrum, it 

 is possible to say that since the maximum at *6 ampere is 

 considerably greater than the effects at '3 and *4 ampere, 

 there must be a length of spectrum beyond the ordinary 

 ultra-violet in which there are very few or no lines, followed 

 by a region in which there are several lines or at any rate 

 considerable energy. The smallness of the effect at *3 and 

 •4 ampere indicates, in fact, a scarcity of lines with fre- 

 quencies between \1300 and A 1800, assuming that the 

 maximum of the photoelectric curve corresponds to a 

 velocity one-half of the maximum velocity due to the 

 exciting frequency. In the same way, the maximum at 



* Franck & Hertz, Verh. der Deutsche Phys. Ges. 1918 — positive ion 

 method ; Bazzoni, Phil. Mag. Nov. 1916 — negative ion method. But 

 see Aston, Proc. Roy. Soc. June 14, 1907, who gets 30 volts hy an in- 

 dependent method. 



f < Nature,' Sept, 7, 1916. 



