446 Prof. F. Horton and Miss Doris Bailey on Measurement 



ionization produced by the helium radiation would not show 

 in the marked way it does in fig. 2. In curves which were 

 taken at various time intervals after the manner of those of 

 fig. 2, the electron velocity at which ionization was first 

 detected decreased gradually from 25 volts to about 21 volts, 

 as the amount of impurity present increased, but the rise in 

 the curve never occurred before about 21 volts, indicating 

 that this is the minimum velocitv at which radiation is pro- 

 duced from the helium atom. With a very small amount of 

 impurity (such as that collecting in a few minutes heating 

 with the tap closed) the current-E.M.F. curve rises at some 

 value of the electron velocity intermediate between the 

 ionization potential difference and the radiation potential 

 difference. This is explained by the facts that the radiation 

 is produced throughout a gradually increasing layer of gas 

 as the accelerating potential difference is raised, and that 

 with a very small amount of impurity a large amount of 

 radiation is necessary to produce sufficient ionization to give 

 an indication on the galvanometer. 



Another instance of the marked effect of a small amount 

 of impurity was obtained in the course of some experiments 

 in which a speck of lime on a platinum strip w T as used as the 

 source of electrons. To make the lime adhere to the platinum 

 it had been mixed with a small proportion of powdered red 

 sealing-wax. The filament had been raised to incandescence 

 by an electric current, and the organic matter burnt up 

 before the filament was sealed into the apparatus, and the 

 heating of the filament had been continued for some days. 

 It was, however, found that with this filament, ionization 

 was always detected at an electron velocitv of about 21 volts, 

 even when the apparatus was in connexion with a carbon 

 tube cooled in liquid air. By examining the spectrum of the 

 luminosity which appeared in the gas at an electron velocity 

 slightly higher than that necessary tor ionization, it was 

 found that the mercury green line X5461 was faintly visible. 

 The small trace of mercury present in the otherwise spectro- 

 scopically pure helium must have come from the vermilion 

 colouring matter in the sealing-wax used, for it was im- 

 possible for mercury vapour to have passed into the apparatus 

 from the pump, as the connecting stopcock had never been 

 open without the intervening U-tube being cooled in liquid 

 air or in solid carbon dioxide. Moreover, with a tungsten 

 filament, the mercury lines were never seen in the spectrum 

 of the luminous discharge when the same precautions were 

 taken to keep the helium pure. 



Fig. 3 is an example of the curves obtained with a 



