74 Proccedmgs of tlie Ro/jal Irish Academy. 



so that the air passed along, about 90 cms. of the tube before entering 

 the field between the charged surfaces. The distance between the top and 

 bottom surfaces and the plate A was 2-1 cms. The plate A was 20 cms. wide, 

 and between it and the vertical sides of the tube were placed strips (C) of 

 glass coated with tinfoil and earthed, so as to leave a space of about 5 mms. 

 between the earthed strips and the plate connected with the electrometer. 

 The object of the strips was to assist in making the transverse field uniform. 

 All the electrometer connexions were led through tubes coated with tinfoil 

 and earthed, so as to shield the system from external disturbances. The 

 capacity of the apparatus and electrometer was "0011 microfarad; and it was 

 generally necessary to introduce another capacity in order to diminish the 

 rate of motion of the spot of light on the electrometer scale. 



The electrometer as usually employed gave about 1,000 scale- divisions for 

 a volt- In most, of the experiments the air was drawn through the 

 apparatus by means of a fan. The end of the tube was fitted into a box with 

 an outlet-tube, in which the fan worked. In this tube was placed also an air- 

 metre, which enabled the flow of air to be measured. In some experiments 

 described later, the flow of air was produced and measured by means of a 

 large reservoir constructed like an ordinary gas-holder. 



The apparatus described was designed after a number of experiments on a 

 smaller scale. We found it more satisfactory to deal with considerable 

 volumes of air, and the large size of the apparatus constituted no objection. 

 It was placed in such a position that, by opening adjoining windows, the air 

 tested was entering directly from a small quadrangle. 



Although this paper is chiefly concerned with the large ions in the 

 atmosphere, we found it desirable to make some measurements of the number 



HI---^HH 



of small ions present. The apparatus used for this purpose is shown on tig. 3. 

 It consisted of a cylindrical tube A, 3-2 cms. in radius and 40 cms. long, 

 insulated and joined to an electrometer, and shielded by an outer earth- 



