Spectra of Helium, Sfc. in the Ultra-Violet. 299 



the pressure of the gas. The pressure must be below that at 

 which the mean free path of the electron becomes of the 

 same order as the distance around from the target to the 

 cup, and it must be above a certain value where the collisions 

 of the bombarding electrons become so few that there is not 

 enough radiation to produce an effect that can be detected 

 on the electrometer. If the mean free path is less than the 

 distance around, disturbances arise from the deflexions of 

 the slow moving electrons by the gas molecules. In the 

 curves shown the pressure was about "06 mm. 



That these curves are due to radiation from the gas and 

 not to radiation from the filament or from the metal parts of 

 the apparatus, is evident from the following considerations. 

 Some of the curves were taken with the anode A : at a low 

 negative potential, the discharge then passing to the anode 

 A 2 , which is out of sight from the target. These curves do 

 not differ in any way from the others. Before admitting 

 the helium, while the apparatus contained a liquid air 

 vacuum, careful observations were made with the purpose 

 of locating any radiation which might be given off from the 

 copper electrodes. Anode Aj was used and voltages up to 

 900 with electron currents up to 10 milliamperes were 

 applied, but no radiation effect was discovered. Curve A, 

 fig. 6, which coincides with the horizontal axis, was obtained 

 at this stage. This was in some ways a disappointing result, 

 as it was thought that radiation from the metal might be 

 expected at these potentials. In these observations the 

 apparatus was, however, not set to indicate radiations shorter 

 than about 60 Angstrom units. Under these conditions the 

 entrance of the slightest trace of gas would produce radia- 

 tion at a point corresponding to the maximum of the curve 

 given at higher pressures. It is proposed to make further 

 experiments on the emission of radiation from solids under 

 similar conditions. 



The lowest driving potential shown in fig. 6 is 200 volts, 

 but similar curves were taken at 100 volts and at 80 volts. 

 The intensity of the radiation at these voltages had fallen to 

 such an extent that the spectrum was shortened so as to end 

 at 1*8 ampere. The apparatus, of course, records only a 

 very small fraction of the total effect due to the radiation, 

 and the total effect itself is extremely small at the limiting 

 frequency. It is proposed to use a more sensitive electro- 

 meter so as to be able to take spectra with driving potentials 

 in the neighbourhood of 30 volts and of 20 volts. 



It is significant that the limiting frequency here deter- 

 mined corresponds very closely, if not exactly, with the 



