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



That the pressure-area increases and the pressure-intensity 



falls as z increases ; 

 That the pressure-intensity is higher for + than for — 

 discharge, the ratio of + to — being greatest at the 

 centre and diminishing to a value not far from unity 

 at the edges of the pressure-area ; 

 That the area over which the current is received is 

 roughly coincident with the area over which the pres- 

 sure is at all considerable ; 

 And that the current-area is slightly greater for — than 

 for + discharge. 

 A similar set of curves was also obtained with the disk D 

 in position, the point being arranged to project about half a 

 centimetre below the surface of D. The maximum value of 

 z in this case was 1*88 centim. and corresponds with the curves 

 marked b. As the curves for very small values of z were not 

 materially altered by the presence of D, curves c may be taken 

 to represent those for the minimum value of zin this case also. 

 This set of curves differs from those taken without D, 

 mainly in the fact that they show a much smaller variation of 

 the pressure and current areas for the same variation of z, 

 and that the maximum pressure-intensity for + discharge is 

 about constant, and for — discharge even increases with 

 increase of z. 



The total current passing between point and plate was kept 

 constantly at 2'37 microamperes throughout all these experi- 

 ments ; this corresponded with 80 divisions of the galvano- 

 meter-scale. 



The following points may be noted in connexion with the 

 curves : — 



The pressures measured cannot depend on the momentum 

 in the ions when they arrive at the plate, since these must 

 strike the plate itself, and not the air in the hole H, in coming 

 to rest. This agrees with the fact that while the pressure- 

 curves for b and c are of the same height, the corresponding 

 current-curves are totally unlike one another. 



The pressure cannot be due either wholly or in considerable 

 part to the imparting of momentum to the gas at the point 

 itself; otherwise the pressure-curves a and b would be alike, 

 since the field at a discharging point is constant for a given 

 current, and the values of z for both these curves are about 

 the same. 



On the other hand, the shapes of the pressure-curves are 

 intimately connected with the shape of the field between point 

 and plate, as is shown by the way in which the pressure-area 

 contracts when the field is rendered parallel by D. 



