598 



Prof. F. Horton and Miss A. C. Davies on 



due to radiation alone. These observations were taken at 

 a pressure of 0'49 ram., so that most of the electrons would 

 not make more than one collision while traversing the distance 

 over which the accelerating potential difference was applied. 

 This curve indicates an increased production of radiation, 

 not at the second multiple of the resonance velocity but at 

 (25-6 + 20-4 = ) 46-0 volts. 



In the curve of fig. 4 the pressure is still lower, being- 

 only 0*17 mm., and at this pressure it is probable that a 



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10 



20 30 



Electron 



70 

 (volts) 



80 



90 



60 

 velocity 



considerable proportion of the electrons pass through the 

 space between the gauzes I) and without making more 

 than one collision with a helium atom, and it is certain that 

 very few, if any, will be able to make three collisions while 

 traversing this distance. For this series of observations the 

 fields were so arranged that radiation and ionization would 

 both cause the electrometer to indicate a positive current. 

 The curve shows a bend at 20*5 volts, another at 25'6 volts, 

 and another one in the neighbourhood of 80 volts. The 

 critical velocity indicated at 80 volts is presumably that at 

 which both electrons are removed from the atom at a sinole 

 collision. Other series of observations agreed in fixing the 

 critical velocity at which this occurs at about 80 volts, but 

 the difficulty of obtaining the point accurately has already 

 been mentioned. This value should be the sum of the 

 velocities required to remove the two electrons separately 

 from the helium atom. We have shown experimentally 

 that an electron velocity of 25 6 volts is required to remove 

 the first electron, so that the difference between the two 

 experimentally determined values, i. e. about 54 volts, is the 

 velocity required to remove the second electron. This 



