. Radiation and Ionization of Helium. 747 



20*1 volts energy, and he states that it cannot be explained 

 by the presence of: a small trace of impurity. Compton's 

 method was devised for distinguishing between an effect due 



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entirely to radiation and one due to both radiation and. 

 ionization. It is essentially a modification of Lenard's 

 method of obtaining the minimum energy required for the 

 ionization of: a gas. The collecting electrode used by 

 Compton was in the form of a cylindrical box, one end of 

 which was covered with platinum gauze, and the opposite 

 end by a sheet of platinum foil. This electrode was so 

 mounted in the ionization chamber that either the gauze 

 end, or the foil end, could be turned to face the source of 

 electrons. This was a glowing tungsten filament situated on 

 the further side of a platinum grid which was in the usual 

 position between the filament and the collecting electrode. 

 The electric fields were arranged in the ordinary way for 

 Lenard's method of experimenting, but observations of the 

 current to the collecting electrode were made first with 

 the foil-covered end facing the filament, and then with the 

 gauze-covered end facing the filament. The ratio, R, of the 

 currents measured in the two cases determines the propor- 

 tions of the observed effect which are due to gas ionization 

 and to photoelectrically active radiation. For pure ionization 

 R should equal unity, whereas for pure radiation R should 

 be equal to the ratio of the outside areas of the collecting 

 electrode exposed to the radiation in the two cases. This 

 latter ratio was calculated by Compton on the assumption 

 that it was equal to the ratio of the area of the closed end of 

 the collecting electrode to the area covered by the wires 

 of! the gauze end. It is on a knowledge of: this latter ratio 



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that the interpretation of the results depends. The value 

 found by calculation was checked by an experimental deter- 

 mination by measuring the photoelectric currents in the two 

 positions of the collecting electrode, using a mercury arc 

 outside the apparatus as a source of radiation. The value of 

 the ratio obtained experimentally agreed with that calculated 

 from geometrical considerations. 



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In investigating the nature of the effects produced in 

 helium at different pressures by electrons with energies 

 between 20 and 25 volts, Compton found that only at low 

 pressures did the observed value of the ratio R approximate 

 to that of: the preliminary determination of its value tor a 

 pure radiation. The value observed was less than the value 

 calculated for pure radiation, and the deviation from this 

 value became greater as the pressure of gas was increased. 

 Compton therefore concluded that the effect beginning at 



a i) 2 



