30G 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE ABSOLUTE 
After the disc had been dismounted, the diameter could be determined more advan¬ 
tageously by direct observation through microscopes focussed upon its edge with 
subsequent reference to the standard scale. It was found (August 19, 1882) that 
a very appreciable difference existed between the diameter of the upper and lower 
faces, showing that the edge was somewhat conical. At the upper edge the diameter 
was 3HP80, and at the lower edge 310’58. These were the extremes. At the middle 
of the thickness the diameter was 31075. This departure from the truly cylindrical 
form was undoubtedly a defect in the apparatus, which could easily have been avoided 
if detected in time. When the apparatus was first set up, the success of the experi¬ 
ment was problematical, and a minute examination of the disc seemed premature. 
The diameter to be adopted is an average taken with reference to the conductivity of 
brush contact. The whole width of the brush being decidedly less than the thickness 
of the disc, and the pressure being greatest at the central parts, we decided (of course 
without knowing to what precise final result the estimate would lead) to take the 
mean of 31075 and \ (SlO’oS + SlO'SO). Thus 
2a= 31072 millims. 
The error due to the conica.lity of the edge cannot exceed one part in 5000 at the 
worst, and thus it appeared scarcely worth while to correct the defect and repeat the 
spinnings. 
The diameter of the shaft at the place where the other brush contact was made, was 
found to be '825 inch, or 20‘96 millims. 
The induction coils. 
§ 23. These are the' same as were used in Mr. Glazebrook’s measurement, and 
were wound by Professor Chrystal in 1878. The following are the dimensions ; for 
further particulars reference may be made to Mr. Glazebrook’s Memoir.* 
A. 
B. 
Mean. 
Mean radius in centims. (A). 
25753 
25-766 
25-760 
Radial width of section (2h) . 
1-92 
1-90 
1-91 
Axial width of section (27c). 
1-896 
1-899 
1-897 
Number of windings. 
797 
791 
4 x 1588 
Resistance (approximate) in B.A. units 
84 
83 
4x167 
Since the coils are so nearly similar and were used symmetrically, it is sufficient to 
use the numbers in the last column. The section of the ring is shown in fig. 4 
full size. 
To find the distance of mean planes the following measurements of the thicknesses 
of the rims are required. They are given in centimetres. 
* Phil. Trans., 1883, p. 223. 
