294 Mr. A. P. Ohattock on the Electrification 



Now these columns show that below a point-radius of r ** 

 millim./ varies in close proportion with r -0 ' 8 , and they con- 

 sequently negative hypothesis (b). But they also negative 

 (c), for the pull per square centiin. normal to a film of sur- 

 face-tension T and radius of curvature r necessary to break it 

 is proportional to T/r, and if this pull is to be supplied by / 

 it follows that/ 2 must be proportional to T/r, which gives 

 — 0'5 instead of — 0*8 as the power of r with which /varies. 

 Even —0*5 is higher than can be assumed, for it is calculated on 

 the supposition that there is no cohesion between the film and 

 the point, and the existence of cohesion would still further 

 reduce the power of r } as tending to bring the film nearer to 

 class (b). 



Resistance to discharge at a point is thus to be found in 

 the surrounding gas, and is therefore practically reduced to 

 the breaking down of Grotthuss chains, the lengths of which 

 are not negligible compared with the radius of curvature of 

 the point. I do not mean by this to exclude surface-resistance 

 as a possible factor in discharge — indeed what follows shows 

 that it may be very appreciable ; but it is not the whole, nor 

 I believe an important part, of the resistance at a clean point. 

 § 5. After a point has been used for discharge for some 

 time its resistance greatly increases, and when discharge 

 occurs it begins with a suddenness and violence which is very 

 suggestive of the bursting of a film. The effect is increased 

 if the point be now reburnished (without regrinding), though 

 this process makes it look as if it had never been used. For 

 instance, Curves II. are those connecting the pressures of 

 the air with / for needle H. The continuous curve A B re- 

 presents / at the commencement of positive discharge, C D 

 being for negative. The readings for these two curves were 

 taken alternately, two at a time, the pressure being gradually 

 increased. They correspond to an unused clean needle. The 

 cloud of points (© for positive and x for negative discharge) 

 was obtained on attempting to repeat the curves, and it was 

 noticeable that each repetition increased the values of/, 

 though the needle was well polished on dry wash-leather and 

 rouge each time. Afterwards the needle was reburnished, 

 and the dotted curves E F ( + ) and GrH ( — ) obtained, show- 

 ing a still further increase of / and no less irregularity in its 

 values than before. 



The above, therefore, furnishes evidence of the growth of 

 resistance at a point when it is used. That the current does 

 not permanently clear the formation away in getting through 

 is obvious from the readings ; but if a reading be taken 

 quickly after another with a point in this condition, the 



