404 Prof. A. P. Chattock on the Velocity and 



to indicate a pressure of about 0*2 dyne per sq. centim. 

 Above A a fine platinum wire B (diam. 0055 millim., end 

 rounded in blowpipe) was carried by the sliding arm Gr, and 

 so adjusted that when moved horizontally by Gr it passed 

 exactly over the hole in A. By means of the scale S it was 

 thus possible to measure the intensity of pressure on A when 

 discharged against by B, for various known distances, S, 

 between the hole and the foot of the perpendicular from B 

 onto A ; the current C received by A being also measured by 

 connecting A to earth through a galvanometer, with special 

 precautions to prevent electricity from passing through the 

 galvanometer other than that received by A from B through 

 the gas. B was arranged to slide vertically, so that the dis- 

 tance z between the point and A could be varied from nothing 

 to about two centimetres ; and the metal disk D could be 

 moved up or down upon the support of B. The edge of D 

 was bound with sheet indiarubber to prevent it from dis- 

 charging. 



The plate close round H was divided into a series of three 

 concentric metal rings (not shown in the figure) which were 

 insulated from one another by ebonite, and of which the mean 

 diameter of the outermost insulation was about 3"3 millim. 

 These rings could be connected separately or with the rest of 

 A to the galvanometer, and were for the purpose of determining 

 the distribution of the current on A simultaneously with the 

 distribution of pressure. As the insulation which separated 

 the rings was less than 0*2 millim. wide, the continuity of A 

 was hardly affected by their presence. 



Curves I. and II. are typical of the results obtained with 

 this apparatus. In each case the abscissas are the values of 

 S (fig. 2) ; the full curves being for positive discharge, the 

 dotted for negative, d is the diameter of the hole in the plate 

 to the same scale as S ; d' , d" , d'", the diameters, to the centre 

 of the ebonite insulation-rings, of the circular areas at H over 

 which the measurements of the current-intensity were made 

 for e, b, and a respectively (strictly speaking d is the mean of 

 the diameters of the smallest and of the smallest but one areas, 

 the corresponding curves c being the means of two sets taken 

 with these two areas). 



In the first experiments the disk D was removed, and a set 

 of curves obtained representing the distribution of pressure 

 and current for values of z varying from 0"20 to 1*91 centim. 

 Of these only the curves a and c, corresponding to maximum 

 and minimum z respectively, are given for the sake of clear- 

 ness. The rest fit in satisfactorily between these two extremes, 

 and show in every case : — 



