764 Prof. F. Horton and Miss A. C. Davies 071 



The positive currents recorded in column II. of the table 

 arise from the bombardment of the gauze C by the electron 

 stream from the lower filament which was maintained at a 

 white heat so as to give a copious supply of electrons. It 

 will be noticed that the magnitude of this positive current 

 rises rapidly with the velocity of the electron stream, so that 

 if electrons lose energy by ionizing helium atoms they will 

 produce a smaller positive current on impact with the gauze 

 electrode. Thus the fact that the currents measured when 

 both filaments were active (column IV.) had either smaller 

 negative values, or larger positive values, than the corre- 

 sponding currents in column III., is evidence that the electron 

 stream was producing ionization in the gas when this was 

 exposed to the radiation from the side filament. The small- 

 ness of the difference between the numbers in columns III. 

 and IV. is, however, evidence that the amount of ionization 

 produced in this way is very slight, for although the 

 ionization of helium atoms which have absorbed radiation 

 causes a reduction in the positive current from electron 

 bombardment of the platinum, which tends to compensate 

 for the production of helium positive ions, it must be noticed 

 that it results also in a reduction of the photoelectric current 

 (negative) due to radiation, for abnormal atoms which were 

 about to radiate are now ionized. 



The energy required to ionize a helium atom which has 

 absorbed energy equivalent to 20"4 volts is (25'2 — 20'4 = ) 

 4*8 volts. Attempts to show that the beginning of ionization 

 of abnormal helium atoms in the main tube (produced by 

 radiation from the side tube) by the impacts of electrons 

 from the lower filament occurred when V 1 was equal to 

 4*8 volts were, however, unsatisfactory. 



The experiments described in this section may be said to 

 indicate that, under conditions such as those employed and 

 for pressures of helium up to 1 5 mm. (the highest pressure 

 used), the effects of electron impacts upon helium atoms 

 which have already absorbed energy can account for only 

 a very small proportion of the ionization of helium pro- 

 duced below the normal ionizing potential difference. 



The ionization of abnormal helium atoms by the 21*2 volts 

 radiation. 



In view of the fact that in the curve of figure 3 the' second 

 critical point was marked by the commencement of a con- 

 siderable ionization current, the effect of allowing radiation, 

 produced by electrons from the side filament, to pass into 



