﻿of the Ionization produced by Rontgen Rays. 117 



deflexion was always compared with that given by the small 

 detector D under the same circumstances. 



It will be noticed that the rays actually fell upon the 

 plates of! the large detector, and that therefore the ionization 

 measured was due not only to the primary rays but also to 

 the secondary rays given off from the plate ; but as this 

 secondary ionization is always proportional to the primary, 

 the result gives a measure of the primary ionization. It 

 is necessary to place this detector transverse to the rays, 

 because the ions collected must all be sensibly the same 

 distance from the anode, which is with this arrangement 

 approximately the case ; for instance, if the wires are 100 cm. 

 from the anode, the ionization measured is that extending 

 from 100 to 101'5 cm., all the ions produced in this length 

 of the beam being collected. 



The various parts of the detectors were held together by 

 ebonite supports, and as additional precaution against leaks 

 these w r ere surrounded in places by tinfoil joined to earth. 

 The remainder of the apparatus shown is an arrangement 

 for eliminating the absorption of the air. It consisted of a 

 long brass tube with a side tube leading to an air-pump ; a 

 shoulder was put onto each end of the tube and two rings of 

 brass fixed to circles of aluminium could be pressed against 

 these. If the surfaces in contact were greased, the whole 

 formed a tube with aluminium ends which could be evacuated. 



First Method of .Experiment. 



The original intention was to arrange the apparatus as in 

 fig. 1, placing the large detector at say 3 cm. from the end 

 of the tube and to compare the deflexions given by the two 

 detectors in 1 minute, the tube being pumped to a calculated 

 low pressure. Air was then let into the tube, which was 

 taken away, the ends removed and placed together as in 

 fig. 2, the left-hand one being in its original position and the 

 large detector still 3 cm. from the other. The deflexions were 

 again compared in 1 minute. From these results could be 

 at once obtained the ratios of the ionizations between the 

 plates of the large detector in its two positions. 



It is obvious that the air absorption can be made the same 

 in the two cases ; for the amount of air traversed by the rays 

 in each case can be made the same by suitably adjusting the 

 first pressure in the tube. Any secondary radiation from 

 the aluminiums would probably be absorbed before affecting 

 the detectors ; and even it not, the relative positions of the 

 left-hand one and the small detector, and also the right-hand 



