﻿351 Dr. G. Owen and Mr. J. H. T. Roberts on the 



The Ionization- Chamber. 

 The ionization- and cloud-chamber A consisted of a 

 cylindrical brass tube about 8 cm. long and 5 cm. in 

 diameter, furnished with two small glass windows through 

 one of which entered a beam of light from a distant In ernst 

 lamp, while through the other could be obtained a view of 

 the cloud. The central electrode consisted of a piece of brass 

 rod about 1 cm. diameter. On account of the repeated 

 production of clouds in our experiments it was necessary to 

 devise means to prevent the insulation of the electrode from 

 being destroyed by the deposition of moisture ; this was 

 secured by supporting the electrode by means of sealing-wax 

 from the upper end of a long narrow^ glass tube B, care 

 being taken that the electrode did not touch the tube at any 

 other point. The electrode was arranged vertically to 

 reduce deposition to a minimum, and the lower end of the 

 insulating glass tube was coated with sealing-wax. Direct 

 leak from the charged tube to the electrometer was prevented 

 by insulating the brass tube C from A, and connecting C to 

 earth. Extraneous ionizing action of the radium was pre- 

 vented by thick lead blocks D, the upper block being 

 tunnelled to receive the tube C. 



For an account of Wilson's expansion apparatus, reference 

 must be made to Wilson's original paper (Proc. Camb. Phil. 

 Soc, p. 333, 1897) or to J. J. Thomson's ' Conduction of 

 Electricity through Gases,' page 154. The descent of the 

 piston (and thus the expansion of the air in A) is brought 

 about by opening the valve Y, thus putting the lower side of 

 the piston in communication with the exhausted chamber Z. 



In our earlier experiments the valve V was opened by 

 pulling the rod T by hand ; since, however, the magnitude 

 of the expansion depends upon the suddenness with which 

 this is done, we found it necessary to arrange for the valve 

 to be opened mechanically. As is shown on the diagram, 

 the necessary force was supplied by the release of a powerful 

 compressed spring P. Further, the pressure in the vacuum 

 was always adjusted to a certain value before an expansion 

 was made. In this way the magnitude of the expansion 

 corresponding to any given pressure- drop w r as made very 

 definite. 



Since the cloud begins to fall immediately it is formed, it 

 is desirable that the measurement of the ionization current 

 be completed within as short an interval as possible after the 

 expansion. On the other hand, when a cloud is formed on 

 the ions, the air is temporarily robbed of its ions and a certain 

 interval must elapse before the ionization once more attains 



