26S Prof. A. P. Chattock on the Forces at the 



the pressure excess upon A above the atmosphere due to the 

 B ions, and p 2 the corresponding reduction of pressure in AC 

 due to the C ions, the resultant pull per square centimetre on 

 A will be 



p = 2>ir<T 2 — P1+P2, 



—pi +p 2 thus representing the change in the pull per square 

 centimetre on the point due to the current discharged 

 from it if the above is a complete account of the pressure- 

 producing part of the process. 



The object of what follows is to compare the magnitude of 

 this change with 2ira 2 . 



Suppose first that the C ions are absent, and consider the 

 effect of pi by itself. 



The ions in the B layer are attracted by A, and the force of 

 this attraction imparts momentum to them, some of which 

 remains in the ions while the rest is transmitted to the gas 

 through which they move. 



Now in the case of ordinary positive point discharge the 

 B ions start very close to A, and it is safe to assume that 

 both parts of the momentum end by being given up to the 

 point in the form of the steady pressure p x . pi is thus equal to 

 the force per square centimetre to which the B layer is sub- 

 jected, viz. 4:7rap — 27rp 2 , and we therefore have 



p = 2no 2 — 4:7T(Tp + 2irp 2 

 = 2ir(*-p) 2 . 



This means that as far as their mechanical effect on the 

 pull is concerned the B ions might just as well have been 

 rigidly attached to A. In other words, if we attempt to 

 calculate the field at the point from the observed pull upon it 

 we shall obtain a value which is less than that of the field at A 

 by the number of lines of force attached to the B ions, and 

 which is therefore due to those lines alone which cross the 

 ionizing region unbroken. 



Next consider the suction effect, p 2 , of the C ions. 



Tn ordinary positive point discharge these ions also start 

 very close to the surface of the metal, but they move on: to 

 distances which are usually large compared with the size of 

 the point. Except in the region near the point, therefore, 

 the momentum they give to the gas is felt as a pressure on 

 any fixed plate or other bodies there may be opposite the 

 point, and does not sensibly affect p.* 



