Mass of the Ions in the Electric Wind in Air 



403 



p makes itself felt as a pressure on A, and may be measured 

 by this pressure if it is due to p alone. This, however, is not 

 necessarily the case. If the ions possess appreciable inertia 

 they will contribute a certain amount of pressure, tt, to the 

 gas, and through it to A, in coming to rest. Hence if P 

 represents the total fluid pressure on A due to the current, 

 (iv.) becomes 



Y=C2/(P-tt) (v.) 



With a sharp point and gas at atmospheric pressure the ions 

 give up the greater part of their momentum very near the 

 point ; when this is the case a curve of P plotted with z 

 should be a straight line cutting the axis of P in ir. 



Experiments by " Hole in Plate " Method. 



The above argument was the outcome of an attempt to 

 interpret a series of observations made at the beginning of 

 last year for the purpose of mapping the distribution of 

 pressure on a flat metal plate when blown against by the 

 electric wind. The method of experiment was as follows : — 



An ebonite box E (fig. 2) was fitted with a false bottom A 



Fijr. 2. 



( Y 



H A 



CZD 



To Gauge 



of brass plate, perforated with a minute hole H (0*9 millim. 

 diam.). H communicated by a tube, T, with a U-tube water- 

 pressure gauge, read by a microscope, and sensitive enough 



2F2 



