292 Mr. A. P. Chattock on the Electrification 



while in (c) it is tangential to the surface only ; i. e. between 

 the particles themselves. An actual case would lie some- 

 where between the two; and probably much nearer to (b) 

 than to (c). 



Now in the case (a) of Grotthuss chains the field / at the 

 point, as measured by the pull upon it, should increase as the 

 sharpness of the point increases : and for two reasons. In 

 the first place, for chains of given length there will be some 

 average strength of field required to break them, the value 

 of which will always be less than that measured at the 

 surface of the point on account of the divergence of the lines 

 of force there ; and the difference between the measured and 

 the average field will be more marked as the divergence (i. e. 

 the sharpness of the point) increases. Secondly, the chains 

 will be shorter at a sharp point than at a blunt one on ac- 

 count of this same divergence ; and this again will necessitate 

 a stronger field to break them, for much the same reason that 

 a short piece of iron is harder to magnetize than a long one. 



The same sort of variation of/ with sharpness of point will 

 occur in the case of (c), if the mechanical pull of the field on 

 the point surface be looked on as having to tear off a film 

 stretched over it. 



But to pull small particles off a point whose radius of 

 curvature is large compared with their diameters will always 

 require the same value of /. Hence (b) differs from (a) and 

 (c) in requiring that /shall be independent of r. 



The effect on / of altering r was therefore investigated. 

 Steel needles were ground by a watchmaker to hemispherical 

 points of different diameters and burnished with a hard steel 

 tool. The values of /in air were then determined for each at 

 various pressures ; the instrument used being the smaller of 

 the two described in § 8. Curves connecting air-pressure (p) 

 and /were then plotted, and from these the numbers given in 

 Table II. were taken. The values of/ for positive discharge 

 are given for three pressures : — 76, 40, and 20 centim. of 

 mercury. Those for negative discharge are only given at 

 20 centim. The reason being that the latter show a good 

 deal of irregularity at higher pressures, seeming (as will be 

 seen later) to depend largely on the condition of the point- 

 surface, which the positive values do not. The values of/ 

 for needle C are inserted with queries, as they are almost 

 certainly too high. The extreme fineness of its point necessi- 

 tated a very rapid tapering to the sharpest part, so that a good 

 deal of the measured pull must have been due to lines of force 

 starting from its sides, This is borne out by the numbers in 

 the four last columns. 



