Field at a Discharging Point. 641 



mechanical force and compared with those derived from 

 the theoretical formula (1). The measurements for various 

 points are conveniently combined by plotting the mean Xa 

 with ap, since Xa is constant when ap is constant. The 

 results are given in curves 2, where the theoretical curve is 

 shown with circles, the experimental curve for positive 

 discharge with crosses, and that for negative discharge as a 

 dotted line. X is measured in E.S. units, a in centimetres, 

 and p in millimetres of mercury. 



Curves 2. 



Xa 



[e.s.unit] 12 



0= Theoretical curve 



«t»= Experimental curve 



for positive point 



• (Dotted line)= ditto 



for negative point 



10 



20 



30 



40 



50 



a >F 



It will be seen that in general the theoretical values are 

 too high, but the agreement is as good as might be expected, 

 the percentage difference being of about the same magnitude 

 as that between the values of minimum discharge potential 

 by which the theory was previously tested by Edmunds. 



Relation between Field and Current. 



It has previously been shown that the field at a positive 

 point in air at atmospheric pressure is independent of current 

 except for a slight initial change at small currents ; and that 

 at a negative point the field appears gradually to decrease 

 with increase of current. It has since been found that these 

 are merely special cases of a more general phenomenon. 



Phil. Mag. S. 6. Vol. 30. No. 178. Oct. 1915. 2 T 



