﻿1160 



Prof. S. Puss and Mr. L. H. Clark 



on a 



In principle, this balance method is similar to that devised 

 by Rutherford and described by Bronson (Phil. Mag. vol. 

 xi. p. 143, 1906). Electrical communication is made 

 between two ionization chambers by joining the insulated 

 electrode which each contains. Initially these electrodes are 

 earthed, but the chambers are maintained throughout at a 

 constant potential difference by connecting them to the 

 terminals of a battery of cells. One chamber is exposed to a 

 beam of X-rays and the other simultaneously to the radiation 

 from a very small mass of radium. Under the action of the 

 two consequent ionization currents, the potential of the 

 insulated electrodes rises to a steady value when a balance is 

 set up. The potential of the electrode is indicated by 

 attaching to it a gold leaf, the deflexion of which is then 

 used to indicate the intensity of the X-radiation. 



Fig. 1 represents a working model on the lines indicated. 



Fig. I. 



A 



B 



Ra.. 



Vessel A supports within it a small ionization chamber 0, 

 measuring 0"4xr0xl'5 cubic cm., the two small opposite 

 ends of which are closed by windows of aluminium foil. This 

 chamber carries along its long axis an insulated brass 

 electrode, connected by a fine wire to the gold-leaf support 

 in vessel B. One wall of this vessel is made of very thin 

 aluminium leaf to allow entry of cl radiation from a small 

 mass of radium. 



Fig. 2. 



Radiumr^ 



XRays, 



A B 



Another form of the apparatus is shown in fig. 2. In this 

 case the ionization chamber C consists of a lead cylinder 

 1*2 cm. long arid 0*8 cm. in diameter fitted with an axial 



