Radiation and Ionization of Helium. 1^1 



filament, F, to the gauze D by means of a potential difference 

 which could be increased or decreased in steps of a tenth of 

 a volt. After passing through the gauze D, the electrons 

 traversed the space DC without acquiring any extra energy, 

 for D and C were maintained at the same potential (V 2 = 0). 

 Between the gauzes C and B the electrons encountered an 

 opposing potential difference, V 3 , sufficiently great to prevent 

 any of them from reaching B. Between B and A a potential 

 difference, V 4 , was applied, a little greater than that between 

 C and B and in the reverse direction, thereby preventing any 

 positive ions originating below the gauze B from reaching 

 the collecting electrode. Thus both the original electrons 

 and any positive ions they may have produced were pre- 

 vented from reaching A, and a current could only be 

 detected when ladiation was produced which acted photo- 

 electrically on B and E causing electrons to travel to A, and 

 the electrometer to indicate a negative current. An example 

 of a curve obtained from a series of observations of the 

 variation of the measured current with increase of the 

 potential difference accelerating the electrons, with the fields 

 arranged in this manner, is given in fig. 2. Following the 

 custom of earlier papers, we shall refer to curves obtained in 

 this way as R curves, since they show the currents due to 

 radiation only. The curve indicates that radiation begins at 

 20' 4: volts, and that at 21'2 volts an increased supply of 

 radiation is produced. This series of observations was taken 

 with liquid air surrounding both the carbon tubes and with a 

 slow stream of helium passing through the apparatus. The 

 result confirms the conclusion of our earlier experiments 

 that, in pure helium, radiation is produced at a minimum 

 electron energy of 20*4 volts, and shows in addition the 

 existence of another critical radiation point 0'8 volt higher, 

 as predicted from spectroscopic considerations. 



In the second method employed for the investigation of 

 the existence of the second critical point and of the effects 

 occurring there, the potential difference V 4 was such as would 

 cause photoelectrons to pass to the collecting electrode as 

 in the first method, but the magnitude of this field was very 

 much smaller than in the experiment just described, so as 

 not to prevent positive ions, produced by ionization of the 

 gas, from reaching the collecting electrode, Thus in this 

 method the photoelectric effect of the radiation would give a 

 negative current, while ionization would result in a positive 

 current being measured. A further difference wns that in 

 the employment of this method V 2 was 0*2 volt applied in 

 the direction to cause positive ions to drift toward- the 



