﻿274 



Mr. R D. Kleeman on the Recombination 



some way on the nature of the ionizing agent. The experi- 

 ments were hegun with X rays. 



§i. 



The X-ray hulb does not, as is well known, give a constant 

 source o£ radiation, and the problem that presented itself at 

 the beginning was how to overcome this difficulty. A null- 

 method was first used. Two ionization chambers were con- 

 nected to an electrometer, so that their leaks were in opposite 

 directions. The potential of one of them, A, was varied, while 

 that of the other, B, was kept constant, but the leak in B could 

 be adjusted and made, equal to that in A by opening or closing 

 a sliding door which admitted the X rays. But this method, 

 since the displacement o£ the sliding door could not be made 

 greater than 5 cms. without introducing different proportions 

 between its parts, was not considered so sensitive as the 

 following : — In this method the leak of the constant potential 

 chamber was stored up, while that of the other chamber was 

 measured directly by means of an electrometer, the stored 

 up charge was then discharged into the electrometer, and two 

 swings of the needle taken. 



The experimental arrangement is shown in fig. 1. The 



Fig. 1. 



chamber DE is the constant potential or standard chamber. 

 AB is the chamber of which the potential was varied, and 

 it will therefore be called the adjustable chamber. The 

 adjustable chamber used for investigating secondary X rays 

 consisted of a circular metal box containing two metal 

 electrodes at a distance of *5 cm. from one another, one of 

 them, ab, was a thick lead plate connected with the mercury 

 cup c, the other, mn, was a thin aluminium plate connected 

 with the battery C. The standard chamber DE was of 



