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



the double ionization of the helium atom at a single collision 

 which, according to Bohr's theory, should occur at 83 volts, 

 would result in the production of less positive electricity 

 than the expenditure of a smaller amount of energy in 

 removing single electrons from three helium atoms, with 

 the consequent Joss of 2b'6 volts velocity at each of the three 

 ^successive collisions. 



In most of the experiments described in this paper, the 

 gauzes D and C were at the same potential, so that the 

 electrons, after being accelerated by the fields Vj and V 2 , 

 traversed the distance between D and C without suffering 

 any reduction of velocity except that resulting from their 

 •collisions with helium atoms. The electron stream was 

 prevented from spreading laterally in this space by means 

 of a magnetic field, parallel to the axis of the tube, applied 

 by passing a current of about 2 amperes through a coil of 

 many turns of wire wrapped round the experimental tube. 

 Since there is no difference of potential between the gauzes 

 D and C, the detection of ionization depends upon the diffusion 

 of the positive ions out of this space. Those which pass 

 through the gauze C will be accelerated by the field V 4 

 (which retards the electron stream) and driven towards 

 the gauze B. Some of these will pass through B and reach 

 the collecting electrode A, provided that the field between 

 B and A does not oppose them, or is insufficient to turn 

 them back. Experiments showed that this method gave 

 satisfactory results, and that, with the gas-pressure suitably 

 adjusted, ionization could always be detected when the 

 accelerating potential difference was increased beyond the 

 ionization value. 



Some of the -curves obtained, which illustrate the differ- 

 ences in the effects occurring at different gas-pressures, are 

 given in figs. 2, 3, and 4 In these curves (and in the others 

 given in this paper) the electron velocities, expressed in 

 equivalent volts, are those of the swiftest electrons present 

 in the stream from the filament. The method of obtaining 

 these velocities for given values of the applied accelerating 

 potential difference is explained in our earlier paper *. The 

 curve in fig. 2 was taken under such conditions that any 

 positive ions produced would be prevented from reaching 

 the collecting electrode, so that the current due to radiation 

 was examined alone. The gas pressure during this experi- 

 ment was about 1*5 mm., which should cause the electrons 

 to make several collisions with helium atoms while traversing 

 the distance over which the accelerating potential difference 

 was applied. It will be observed that the curve indicates an 



* Loc. cit. 



