of the Ionization Velocity for Electrons in Helium. 445 



this curve breaks away from the straight line at a lower 

 value of the electron velocity than curve I. An hour later 

 the observations were repeated, and, as will be observed 

 from carve III., ionization was then detected at an electron 

 velocity of about 21 volts. After these observations the gas 

 was withdrawn from the apparatus and made to circulate 

 repeatedly through carbon cooled in liquid air, the gas being 

 thus re-purified ; on again testing, ionization did not occur 

 until the electron velocity had been increased to 25 volts. 

 These experiments show that the detection of ionization at 

 electron velocities less than 25 volts is due to impurity, 

 which can be removed from the helium by repeated circu- 

 lation through carbon cooled in liquid air. 



It will be noticed that in curves II. and III. of fig. 2 in 

 which the indication of ionization occurs before 25 volts, 

 there is no indication of additional ionization when 25 volts 

 is reached. This shows that the direct ionization of the 

 helium by 25 volt impacts does not result in the production 

 of more ionization than the indirect ionization of the impurity 

 by the helium radiation. 



The amount of impurity collecting in the apparatus under 

 the conditions indicated must have been very small in view 

 of the long treatment to which the apparatus had been sub- 

 jected before these observations were begun, and it might 

 seem unlikely that so small an amount of impurity should 

 cause so marked an effect in the current-E.M.F. curve. It 

 must be observed, however, that when the helium is pure, 

 ionization is caused only by electron collisions, and is 

 therefore limited to that part of a column of gas in the 

 path of the electron stream, in which the electrons collide 

 with atoms with enough velocity to ionize them, while with 

 the impure gas the impurity is ionized by the helium 

 radiation, so that in the presence of sufficient radiation, 

 ionization can take place throughout the whole volume of 

 the gas in the apparatus, thus giving a considerable current 

 through the galvanometer when only a very small percentage 

 of impurity is present. 



The same considerations also explain why the ionization of 

 the impurity by the radiation from the helium atoms is 

 marked when there is no indication of the direct ionization 

 of the impurity by electron impacts, although the impurity 

 must be ionizable by collisions with electrons having less 

 than 20 volts velocity, since it is ionized by the helium 

 radiation. If there were enough impurity present in the 

 helium for an indication of its ionization by electron impacts 

 to be given in the curves, it is probable that the extra 



