of Steel Needle-Points in Air. 291 



from it bigger than that from V, whereas the reverse is the 

 case. It is true that the density must be greater at the centre 

 of the point than at the sides of the hemisphere. But this is 

 what P chiefly depends upon, and it is also precisely here that 

 discharge occurs and the value of / is required. Hence the 

 connexion established above between P and / seems to be 

 practically true, although the assumption that / is uniform 

 over the point-surface is not. 



In what follows, therefore, the values of / have in every 

 case been determined from P and r. 



Effect of Curvature of Discharging Surface. 



§ 4. Two conclusions appear, then, to result from the fore- 

 going. In the first place, the absence of discharge below a 

 certain definite potential, which is constant for given con- 

 ditions, implies the breaking down of some resistance before 

 electricity can pass off at a point ; and, secondly, the fact that 

 / is independent of d points to the near neighbourhood or 

 actual surface of the point as the seat of this resistance. 



All this is in accordance with what is known regarding the 

 discharge of electricity in gases, and is indeed only what one 

 would naturally have expected. It still remains to consider 

 whether the resistance to discharge is to be sought in the gas 

 itself, in the surface of the point, or in both. 



The possible ways in which resistance may arise are, I 

 think, all to be included under three heads : — 



(a) It may exist in the gas itself, to a greater than mole- 

 cular distance from the point. For this the gas must become 

 possessed of something in the nature of structure round about 

 the point (t. e. cease to be gas in the ordinary sense of the 

 word), and the only kind of structure that suggests itself as 

 likely is that of the polarized Grrotthuss chains which Prof. 

 J. J. Thomson has used with such effect in his beautiful theory 

 of strise. 



(7>) It may exist at the surface of the metal only, and con- 

 sist in the tearing away from the point by electrical force of 

 electrified particles which are clinging to it (a special case of 

 which is the forcing of charged particles through a non- 

 conducting layer on the point) . 



(c) Or it may mean the pulling off of something in the 

 nature of a film possessing surface-tension. 



As a matter of fact (b) and (c) are two opposite extremes 

 of the same phenomenon. In both, particles adhering to the 

 point are pulled off — but the adhesion of (b) is normal to the 

 surface only, i. e. between the particles and the metal ; 



