CH. VIII.] 



OF AN ELECTRON 



85 



To Earth. 



To 

 Eiectrometer. 



FIG. 12. A represents one of the vessels in which the fog is formed 

 whose rate of fall is to be measured by Mr. Wilson's method : it is 

 arranged for the ionisation produced by X-rays. The vessel A, con- 

 taining some water, and covered by an earthed aluminium plate, is 

 in communication with a vessel C through the tube B. Inside C is a thin- 

 walled test-tube P, which serves as a piston or bell-jar (shown rather 

 too stumpy) and with its lip always dipping under water like a gasometer. 

 D is an indiarubber stopper closing the end of tube C : the lower part 

 of the tube C ought to be shown filled with water to such a height that 

 the mouth of the piston is always below the surface. A glass tube con- 

 nects the inside of the test-tube P with a space E. The space E may 

 be put in connection with an exhausted s^ace F through the tube H. 

 The end of the tube H, inside the space E, is ground flat, and is closed 

 by an indiarubber stopper I, which is kept pressed against the tube H 

 by means of a spiral spring. The stopper I is fixed to a rod K ; by 

 pulling the rod down smartly the pressure inside the test-tube is 

 lowered, and the piston P falls rapidly until it strikes against the 

 indiarubber stopper D. The falling of the piston causes the gas in 

 A to expand : the tubes R and S are for the purpose of varying the 

 amount of the expansion. Before an expansion the piston P is raised 

 by admitting air through T, which is then closed. Then, when every- 

 thing is ready, K is pulled, and the cloud forms in A. 



