302 



Mr. A. P. Chattock on the Electrification 

 Table IV. 





r. 



k. 



c 



0-7 Xl0~ 3 



16 „ 



4-03 „ 



4-84 „ 



6-52 „ 



7-11 „ 



7-83 „ 



8-71 „ 



109 „ 



31-8 „ 



45-7 „ 



58-0 „ 



4000 „ 



1-36 

 1-42 

 1-40 

 1-26 

 1-28 

 1-23 

 1-24 

 1-32 

 113 

 108 

 0-94 

 0-94 

 0-83 



D 



F 



a 



H 



I 



J 





K 



L 



M 



N 



o 



x 



These are given in Table IV., where k is the ratio at a pres- 

 sure of 20 centim. of/ for positive discharge to /for negative. 

 The ratio shows a distinct tendency to decrease as r increases. 

 It is almost useless to give values at higher pressures on 

 account of the uncertainty as to the condition of the point, 

 and the great effect this may have on the negative discharge 

 (Curves III.) ; but the ratio is distinctly less at higher 

 pressures for points which seemed quite clean : the decrease 

 of k at 70 centim. mercury varying from 3 to 8 per cent, as 

 compared with k at 20 centim. For dirty points the decrease 

 may reach 50 per cent. 



Description of Apparatus. 



§ 8. The electrometer used for potential measurements was 

 constructed on the principle of the one designed some years 

 ago, for absolute measurement of much smaller quantities, by 

 Prof. Minchin. The attracted disk is suspended, and the force 

 of attraction measured by tilting the whole instrument until 

 the disk falls back by its weight to a fixed point. The disk 

 is a sheet of mica, covered on one side with tin-foil and metal- 

 lically connected with the case of the instrument, which is a 

 tin cylindrical box 30 centim. long by 30 centim. diameter. 

 The disk hangs just outside a 3-inch hole in a tin screen at 

 one end of the case, and opposite the hole inside the case is 

 an adjustable disk of tin which forms the attracting plate. 

 The sensitiveness of the instrument may be varied very greatly 

 by altering the distance between this plate and the suspended 

 disk. This arrangement does not of course permit of absolute 

 measurement, but it was thought that calibration in terms of 

 spark-lengths was sufficiently accurate for the work in hand. 



