﻿Balance Method of measuring X-Jtays. 1161 



electrode. Each end of the chamber is closed by a very thin 

 mica window coated with aluminium leaf. A lead diaphragm 

 0'5 cm. in diameter carrying a minute lead disk at its centre 

 allows an annulus of radiation to enter the chamber C, and 

 pursue a direct air path to the further window, without 

 impinging upon either the interior of the chamber or the 

 axial electrode. 



Under working conditions 200 volts are applied to the 

 chamber C, the connecting tube and the vessel B are earthed, 

 the central electrode and gold leaf being initially earthed — 

 then left insulated. The deflexion of the gold leaf is 

 observed through a tele-microscope or the image of the leaf 

 is projected on a screen. 



From a practical point of view it is important to know 

 how small a quantity of radium can serve to balance the 

 ionization current due to a powerful beam of X-rays. In 

 the two types of apparatus shown diagrammatically in figs. 1 

 and 2, 008 mgm. of radium bromide served to balance 

 currents set up in the given ionization chambers C, when the 

 latter were approximately 30 cm. from the anticathode of a 

 Coolidge tube. The radium was spread over a circular 

 area of 2 cm. diameter, the preparation being covered by 

 a thin layer of mica, which allowed the a, rays to pass 

 through. 



With the ionization chamber C set up at a definite distance 

 from the anticathode of a Coolidge tube, a series of obser- 

 vations was made to determine the way in which the 

 deflexion of the gold leaf varied with the intensity of the 

 X-radiatiou from the bulb. Keeping the spark-gap constant, 

 the filament current to the tube was varied over wide 

 limits and the intensity of the X-radiation was measured by 

 a distant electroscope. Simultaneously with these readings 

 the balance deflexion was observed. 



The curve of fig. 3 was obtained with the apparatus shown 

 in fig. 2. It indicates a simple relationship between the 

 balance deflexion and the intensity of the X-radiation 

 entering the chamber C. The conditions of the experiment 

 were as follows: — 180 volts on chamber C, distance of C 

 from anticathode 27 cm., alternative spark about 4 inches : 

 the distance of the radium from the aluminium window 

 in B, 2*2 cm. 



The time taken for a state of balance to be set up was 

 found to be independent of the intensity of the X-ray beam. 

 If the leaf be started from its zero position it moves more 

 slowly to its position of balance with a weak than with a strong 

 intensity, but the smaller actual displacement practically 



Phil. May. S. 6. Vol. 44. No. 264. Bee. 1922. 4 F 



