270 Prof. A. P. Chattock on the Forces at the 



For positive discharge in air at atmospheric pressure I find 

 that the field / at the centre of a hemispherical point when 

 discharge is just ceasing is given by the empirical formula 

 f Q r®' A!i = constant, 



where the constant as corrected by Young (Joe. cit.) is 85 if 

 r is in centimetres and/ in E.S. units*. 

 Hence 



tr=18/47r< 45 , 



and p 9 _ C 0-0035 



The largest current used in the experiments referred to 

 was about 15 microamperes, and the ]argest value of r was 

 0*062 cm. V for positive discharge in unit E.S. field is 

 400 cm. sec. -1 , and il was roughly 2-7T judging by the area 

 of the glow. 



k for these data is 0'12; and, for the smallest point of radius 

 0-004 cm., /c = 0'14. 



As the area of the point surface at which discharge occurred 

 happened to coincide with that upon which elective pull 

 (i. e. pull with a component parallel to the axis of the point) 

 was exerted by the field, these values of k give the ratio of 

 the total axial suction effect of the C ions to the total axial 

 pull of the field on the assumption that both o 2 and p 2 were 

 similarly distributed over the discharge area. 



a 2 was probably uniform (see below), but as the current 

 density must have varied from zero at the edge to a maximum 

 at the centre of the discharge area, p 2 must have varied in a 

 corresponding manner. r lhe exact law of this variation we 

 have no means of knowing, but we may obtain an idea of the 

 sort of error introduced by assuming p 2 uniform, if we adopt 

 some arbitrary law: say p 2 proportional to sin « (fig. 2). 



Eemembering that the total suction normal to the surface 

 of the point will be the same whatever the law, this particular 

 law leads to an axially resolved suction equal to four-thirds that 

 for uniform distribution of p 2 . In other words, if we take 



£=0-14 x 4/3=0-19, 



we shall correct for the want of uniformity in the distribution 

 for this particular case. 



* This power of r and the value of the constant were obtained recently, 

 and differ considerably from those aiven in my original paper (loc. cit.). 

 The difference is due to the tapering of the sewing-needles used in the 

 earlier measurements, the effect of which upon the pull was unwarrant- 

 ably neglected. The later measurements were made upon platinum 

 wires with their ends rounded to hemispheres in the blowpipe. 



