the Natural Ionization in a Closed Vessel. 219 



diameter, through which passed an ebonite stopper. A brass 

 wire terminating in a brass cup, C, fitted centrally into the 

 stopper. The wire was fastened by means o£ a metallic 

 spring to a rod K' similar to K, at the bottom of the pressure- 

 chamber. A sulphur plug in C supported the gold-leaf 

 system, which could be viewed by means of a reading micro- 

 scope, through two windows in the electroscope which could 

 be placed in line with two others in the pressure-chamber, 



The electroscope could be charged from the outside and 

 the leaf left insulated as follows : — A brass wire passed 

 through the ebonite stopper D, and was bent so that by 

 moving it could be brought into contact with the gold-leaf 

 system. The wire is shown at EEE, fig. 2. It was attached 

 by a spring F to an ebonite plug and to the terminal K by a 

 wire H, which passed through a copper ring J fixed into 

 another ebonite plug. By raising the rod K, the wire E 

 could be brought into contact with the gold-leaf system, and 

 the leaf charged by connecting K to one end of a battery of 

 100 small storage-cells, the other end of which was to earth. 

 On lowering K, while charged, the wire E was drawn back 

 by the spring F, and the gold-leaf remained charged. In 

 each experiment the rod K was lowered by the same amount 

 and then connected to earth, in order to get the same electro- 

 static conditions each time. The pressure of the air was raised 

 by connecting the pressure-chamber to a cylinder which had 

 been charged to 100 atmospheres by the compresser of a 

 liquid-air machine. The air passed into the pressure- 

 chamber through two stopcocks, by means of which its rate 

 of admission was regulated. It had to be allowed to enter 

 very slowly or the gold-leaf was torn off its support. The 

 air entered the electroscope through the copper tube T 

 (fig. 2), which ended in a flat disk 2 inches in diameter, 

 placed as shown. By this means the air was prevented from 

 impinging directly on any one portion of the leaf and so 

 injuring it. 



Experiments with y Rays. 



It will be more convenient to consider first some experi- 

 ments on the effect of pressure on the ionization produced by 

 y rays. The experiments were made simultaneously with 

 those on the natural leak, 1*09 m.g. radium bromide being 

 fixed to a stand which could always be placed in a definite 

 position outside the pressure-chamber by means of a hole 

 and slot clamp. The measurements of the ionization were 

 made by noting the time taken by the image of the gold-leaf 

 to move over a 'certain number of divisions of the scale in 

 the eyepiece of a reading-microscope. 



