168 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
(d) The Determination of x, the Mean Distance between the Centres of Symmetry of 
the Large and the Small Coils. 
The distance x can be most accurately determined by measuring the total distance 
between the shoulders at each end of the supporting tube, and also the thickness of 
the frame of the small coils where it fits against the shoulders. The reversing and 
replacing of the small coils during a complete series of observations for lo/f eliminated 
errors due to any asymmetry (in the direction of the axis) in the position of the frame 
or of the supporting tube with regard to the centres of the two systems of coils. 
The following series of twenty readings taken at regular intervals round the 
shoulders was obtained with large calibrated vernier calipers* :— 
Table VII. 
Distance between shoulders. 
Temperature of tube. 
cm. 
° C. 
60-720 
24-5 
20 
09 
20 
20 
20 
22 
00 
20 
20 
22 
22 
20 
20 
22 
22 
22 
20 
20 
22 
24-9 
Mean . . . 60-7209 
24-7 
Hence at 20° C. the distance reduces to 60‘7158 cm. 
This number was checked by two similar sets taken at 24°‘0 C. and 22°‘0 C. 
The thickness of the coil frame where it fitted against the shoulders was determined 
by means of a Sweet Measuring Instrument and standard distance pieces. The 
following figures are each a mean value of a series of ten observations taken round the 
parts which fit against the shoulders :— 
* The mean of the observations, rather than the maximum, is chosen because this variation depended 
on the setting of the calipers. 
