/12 MR. J J. THOMSON ON THE NUMBER OF ELECTROSTATIC 
The dimensions of this condenser were ascertained in the following way :—The 
length of the cylinder was measured by beam compasses, and the diameters of the 
inner and outer cylinders by callipers; the difference of these readings was not, 
however, taken as the distance between the cylinders, fur though the error made in 
determining the diameter of either cylinder may be a small fraction of either diameter, 
yet since the diameters are nearly equal, it may not be a small fraction of their 
difference. The distance between the cylinders was determined by fastening the 
middle pieces of the two cylinders down to a flat board by a thin layer of shellac, and 
then filling the space between them with water which had been boiled a few hours 
before the experiment so as to be in a condition to absorb any air-bubbles that might 
be formed. The quantity of water required to fill this space was carefully weighed. 
This gives the volume of the water, and knowing the length of the cylinder and the 
diameter of one of them, the difference of the diameters can be calculated. 
The results of these measurements are :— 
Length of cylinder, measured by beam compasses. 
GO-97 
60-965 
60-97 
Mean 60 "9 6 8 centims. 
Internal diameter of outer cylinder, measured by callipers. 
9-986 
9-989 
9-992 
Mean 9"989 inches, or 25‘372 centims. 
External diameter of inner cylinder, measured by callipers. 
9-254 
9"255 
9-250 
Mean 9"253 inches, or 23"50 centims. 
Weight of water required to fill the space between the cylinders. 
4406"8 grammes at 17’5° C. 
4404-6 „ 13-5° 
4401 „ 12-2 
4403 ,, 11-5 
Mean 4 4 0 5 • 1 grammes. 
