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



The keys F and G could be dropped by releasing the 

 spring catches by the small electromagnets shown at the 

 sides : thus the keys were never touched by hand, and con- 

 sequently all operations could be reproduced identically. 

 The electrometer and keys were enclosed in earthed metal 

 boxes. These arrangements were found to work very 

 satisfactorily ; measurements of the ionization current in 

 cloudless air concorded within considerably less than one 

 half per cent. During the experiments the radium was used 

 in one or other of two standard positions, R x and R 2 in fig. 1- 

 The ionization inside A with radium in position R x was six 

 times as intense as in position R 2 . It remains to add that 

 during any one set of observations the air inside the apparatus 

 was never changed, the clouds being removed by repeated 

 small expansions. 



Experiments. 

 With this apparatus we have investigated the following : — 



(1) The effects of clouds of constant density upon the 



ionization current under different intensities of 

 electric field. 



(2) The effects of clouds of various densities upon the 



current under a constant electric field. 



(3) The manner in which the effect of a cloud upon a 



current varies with the age of the cloud. 



(1) ihe effect of clouds of constant density upon the ionization 

 current under different intensities of electric field. 



In Table I. the first column gives the potential on the 

 ionization-chamber. The second column gives the current 

 through cloudless air at a pressure of 58'3 cm. of mercury ; 

 for since the current depends upon the pressure, it is clear 

 that, in order to compare the currents through cloudless and 

 cloudy air, the current must in all cases be measured with 

 the piston of the expansion apparatus in its lowest position, 

 when the pressure in the ionization-chamber is less than 

 atmospheric by an amount equal to the pressure-drop for 

 this expansion. Column 3 gives the current through a cloud 

 formed on the ions produced by the y rays from the radium, 

 aud caught by an expansion of 1'32. It must be noted, 

 however, that there was no electric field on when the 

 expansions were made ; otherwise, since various fields were 

 applied, the clouds produced upon the ions would not have 

 been of constant density, owing to the different numbers of 



