68 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
holder, and good contact was ensured by a spring attached to the supporting ring and 
brush, which forced the latter towards the micrometer screw. The brush wires were 
put under considerable tension and petrol was used as a lubricant. Measurements 
were made at speeds varying from 170 revolutions per minute to 1110 revolutions per 
minute, the speed being registered by the directly driven chronograph. The results 
of three sets of measurements are plotted in fig. 14. The relation between the 
O 2 4 6 S IO 12 14- IS 
N 2 X, 10' 5 
N = N° OF REVOLUTIONS PER MIN 
Fig. 14. 
increase in radius and the square of the number of revolutions per minute is 
dr = 37 n 2 x 10~ 8 , where dr is the increase in radius in millimetres and n is the 
number of revolutions per minute. The normal speed of the apparatus when resist¬ 
ance measurements are in progress is about 1050 per minute, and the mean radius is 
then 0'04 mm. greater than the mean value given in Table VIII. The effective 
radial distance at 20°'0 C. of the segments is therefore 26'787 + 0‘004 = 26'791 cm. 
for disc No. 1 and 26'776 + 0'004 = 26‘780 cm. for disc No. 2. 
Section 17.— Determination of the Distance between the Centres 
of the Coils. 
The distance between two coils on opposite sides of a disc has to be known with 
considerable precision, as a change of three-thousandths of a millimetre in this distance 
changes the mutual inductance of the coils and discs by 1 part in 100,000. As it is 
not possible to make direct measurements of this distance with both rapidity and 
precision, we followed the plan adopted by Glazebrook* (and by Lord Rayleigh in 
1882) of reversing the coils, without interchange, and then repeating the resistance 
* ‘Phil. Trans./ vol. 174, p. 254, 1883. 
