688 Mr. C. G. Barkla on Secondary 



case, the leak through the air was of course the same ; and 

 when the difference of potential between the supporting rod and 

 the gold-leaf and plate was the same, the leak by the support 

 was also the same. 



The potentials of the rod, plate and gold-leaf, and the case 

 may in the two experiments be written 2?i, n, 0, and 0, n, 

 respectively. 



The leak by the support was thus -J(# + /3), and the leak 

 through the air -J(/3 — a). 



The support leak, however, in the following experiments 

 was to the leaf and plate from the supporting rod, and was 

 excessively small as the potential of the leaf and plate was 

 not allowed to sink much below that of the supporting rod. 



Any leak then must have been due to the conductivity of 

 the air in the electroscope. In its steady normal state, when 

 the potential was high enough to produce a saturation 

 current, this measured the spontaneous ionization of the air. 

 Anything more than this must have been due either to the 

 introduction of ions from outside, or to the formation of ions 

 in the inclosed air itself by radiation from outside. 



In the preliminary experiments made upon air which was 

 not inclosed, care was taken that the beam of X-rays emerg- 

 ing from the second screen did not fall upon any solid in the 

 neighbourhood. It was shown by putting screens much 

 nearer and measuring the secondary radiation from these 

 that the secondary radiation from the wall (distant 2 metres), 

 upon which the beam fell, was inappreciable. The electro- 

 scope was placed well out of the direct beam (through the 

 two rectangular apertures in the box and screen) with a thin 

 paper face parallel to the near bounding-plane of the beam. 

 The electroscope-case, box, and screens were earthed. 



The rate of motion of the gold-leaf was noted when the 

 induction-coil was not working, so that this measured the 

 spontaneous ionization of the air. When the bulb was worked 

 the leaf fell much more rapidly. 



The aperture A (fig. 2) in the box was then covered with 

 lead, and the effect on the electroscope was again normal, 

 showing that the rapid fall was not due to direct radiation 

 through the lead covering of the box inclosing the bulb and 

 induction-coil and through the second opening B to the paper 

 face of the electroscope. The effect was therefore caused 

 by radiation proceeding through A. 



When B was covered with lead the motion of the gold- 

 leaf was again normal,, so that the fall was not due to direct 

 radiation from A through the lead of the screen. 



The electroscope being out of the direct beam, the leak was 

 due indirectly to radiation passing through A and B. 



