IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 



255 



were obtained from a suitable number of small lead accumulators, the potential 

 difference afforded by each being approximately 2 '02 volts. 



The Rontgen-ray bulb was enclosed in a lead-covered box with apertures of 

 dimensions sufficient to allow the rays to have access to the chambers. 



For a more detailed account of the essential features of the arrangement the 

 reader is referred to LANGEVIN'S original memoir. 



P' 



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I 



Fig. 5. 



The lonisation Chambers. The two ionisation chambers represented by AB and 

 A'B' in fig. 4 were exactly identical in structure and dimensions ; a longitudinal 

 section of one of them is shown in fig. 5. The upper electrode A was a brass disc of 

 diameter 5 - 91 cm., and was surrounded by a brass guard-ring G, the outer diameter of 

 which was 9 cm. A and G were screwed on to an ebonite disc, and A was connected 

 to the electrometer by means of a wire led out through an ebonite plug. The 

 guard-ring G and the electrode A were separated from the lower electrode B, which 

 was of aluminium, by means of an ebonite ring perforated in several places to allow 

 the gas or vapour to have free access to the space between the electrodes. The 

 distance between A and B was exactly 1 cm. B was connected with the point b of 

 the key Z (fig. 4) by means of a wire led out through an ebonite plug, and was kept 

 tightly in position by means of the ebonite ring rr. The bottom of the chamber 

 consisted of an aluminium disc together with an outer covering of thick lead with 

 a central aperture of diameter 3 '56 cm. The Rontgen rays passed through this 

 aperture in the lead and through the aluminium constituting both the bottom of the 

 vessel and the electrode B ; the gas or vapour between the electrodes was thus ionised, 

 and in addition there was strong secondary ionisation produced in the layers of gas 

 or vapour in the vicinity of the electrode A. The effect of the outer lead covering 

 was to confine the ionisation to the central portion of the interval between the 

 electrodes A and B. 



