of Radiation by Electron Impacts in Helium. 559 



is applied to the charcoal tubes. The pump and the copper- 

 oxide spiral for removing traces of hydrogen have been 

 previously described*. The spectrum, when excited by a 

 discharge between incandescent tungsten electrodes, showed 

 no impurities in the helium except a trace of neon, whose 

 strong lines appeared faintly when an intense arc was set up 

 between the electrodes. These neon lines, three in number, 

 were of about the same intensity as the individual lines into 

 which the band-spectrum of the helium could be resolved, 

 and were therefore extremely faint. This, in connexion with 

 the well-known effect of helium in accentuating the spectra 

 of any impurities which may be mixed with it, shows that 

 the helium was very pure. Its spectrum excited by a 

 Geissler-tube discharge in a baked-out tube showed no lines 

 except those of helium. A two-stage diffusion pump, with 

 an oil- pump backer, is used for the initial exhaustion of the 

 apparatus. 



The electrometer arrangement is so satisfactory as to 

 warrant particular mention. The electrometer sensitiveness 

 may be varied quickly from zero to 10,000 mm. per volt by 

 varying the potential of the needle. The electrometer key 

 is designed either to insulate the quadrants and electrode E, to 

 earth them directly, or to earth them through india-ink resist- 

 ances of 3*15 (10 8 ) ohms, 3*75 (10 9 ) ohms, or 2'37 (10j 10 ohms 

 as desired. It is therefore possible to pass instantly from one 

 sensitivity to another, and to measure all currents by the 

 steady deflexion method. The india-ink resistances are found 

 to be constant and to show no polarization. A galvanometer 

 is introduced in the connexion to the gauze Gr in order to 

 measure the total electronic current I. 



In preliminary experiments large ionization currents setting 

 in at about 11 volts, and more strongly at about 15 volts, 

 masked the effects due to helium. These were caused by 

 water-vapour, which was slowly given off by the ghiss. This 

 disturbing effect was eliminated by surrounding the ionization 

 tube and the two charcoal tubes in electric heaters and main- 

 taining them at 300° to 350° C. for about a week with the 

 vacuum-pump in operation. The ground joint J was sealed 

 with Khotinsky cement and was water-cooled during the 

 " baking-out " treatment. During the later stages of heating, 

 the filament was kept at a white heat to drive off occluded 

 gases. Liquid air was placed around the mercury trap before 

 these heaters were removed, and was kept there continuously 

 throughout the course of the experiments. 



* Benade and Compton, U>c. rtf . 



