II. b. Methods and Results of Experiments with Stream 

 of Falling Shot or Beads. 



In all the above experiments the bullet was of lead. 

 Attempts to make bullets for the air-rifle of other material 

 were not successful, and since it was desirable to investigate 

 fully the influence of the nature of the impinging material 

 other methods of producing the effect were sought. After 

 tentative experiments by firing an arrow tipped with different 

 materials and dropping bodies from a height upon a plate, in 

 all of which a slight ionization in the immediate vicinity of 

 the target was detectable, we were finally led to the construc- 

 tion of the apparatus sketched in fig. 5, by means of which a 

 steady stream of shot or similar material was allowed to fall 

 upon a solid plate. A steady stream of air drawn over the 

 plate could then be examined, and was found to contain, 

 under given conditions, a remarkably constant number of 

 ions. The order of magnitude of the effect in the case of 

 lead shot was such that one E.S. unit of charge was collected 

 per 3 x 10 8 ergs of kinetic energy destroyed. This, however, 

 can be regarded only as a very rough approximation, owing 

 to the difficulty of getting a sufficiently rapid current of air 

 to prevent loss by recombination and for other obvious reasons. 



The shot is contained in a copper vessel, A, and falls 

 freely, when a shutter covering .the hole in its bottom is 

 drawn aside, down a brass tube, B, from this into a wider 

 tube, C, at the base of which is the target inclined at 45° to 

 the horizontal. The shot by an aperture at 1) escapes into the 

 vessel E. A current of air is drawn by means of a filter 

 pump over the target and either through a Zeleny tube, F, or 

 through a Faraday tube (fig. 5a), which serve respectively to 

 measure the total ionization of a given sign or mobility of the 

 ions and the total charge. The outline, P, in fig. 5a represents 

 a plug of cotton wool. The height of the vessel, A, and the 

 material of the target can be varied at will. 



The first experiments with this apparatus were directed 

 to a confirmation of the law that the ionization is propor- 

 tional to the energy of impact, i.e., in this case, to the height 

 of fall. The accuracy with which this law holds for the impact 

 of a stream of leaden shot on a brass target is shown by fig. 6. 



The mobility of the ions was determined by Zeleny's 

 method (J. J. Thomson, Conduction of Electricity through 

 Gases, 2nd Ed., p. 58). The saturation curves for the positive 

 and negative ions respectively are shown in fig. 7, a and b. 

 The values of the mobilities when the abscissae of the points 

 X and Y were used in calculating were l'GxlO" 5 cm. /sec. 

 per volt /cm. for both positive and negative ions, but the 



