158 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
We may therefore take 27‘4105 cm. as the equivalent of a reading 0'8000 at 
25°'0 C. 
A sample set of measurements for determining the distance between the fixed coils 
is given below. In every case where cl is recorded in this paper the value is the 
mean of a set of observations taken in this way. The bracketed figures give the 
temperature of the extension calipers. 
Table III.—Micrometer readings in inches. Dynamometer at 20°'0 C. 
At 1st point. 
At 2nd point. 
At 3rd point. 
At 4th point. 
At 5th point. 
0-8074 (25-1) 
0-8255 (24-6) 
0-8215(24-2) 
0-8125 (23-6) 
0-8168 (23-0) 
76(25-0) 
45 (25•3) 
24 (25-4) 
25(25-4) 
72 (25-4) 
80 (25-0) 
45(25-0) 
22 (24-0) 
26(24-4) 
65 (24-8) 
80(24-3) 
48(24-8) 
13(25-3) 
22(25-2) 
72 (25-0) 
81 (24-3) 
57 (24-8) 
27 (25-3) 
20 (25-2) 
75 (25-0) 
Mean 0-8078(24-7) 
Mean 0-8250 (24-9) 
Mean 0-8220 (24-8) 
Mean 0-8124 (24-8) 
Mean 0*8170 (24-6) 
The mean for the five points is 0 f 8168 at 24 0, 8 C. 
Now since we know that a reading of 0'8000 inches corresponds to 27'4105 cm. at 
25~'0 C. it follows that 0'8168 at 24°'8 C. corresponds to a length of 27'4530 cm. and 
the mean thickness of a coil is 2'5556 cm., hence 2d = 24'8974 cm. and we have 
d — 12'4487 cm. in this case. 
Five measurements were taken with micrometer calipers at each of the five points 
on each of the fixed coils to determine the thickness of the coil frames, and a value of 
2'5556 cm. was obtained for the average thickness of a coil frame at the points used 
for measurement. 
All the deflection observations were reduced to the value for 
d = 12A448 cm. at 20 C. 
IV. The Batio of the Badii of the Fixed and Suspended Coils. 
(a) The Method. 
The method employed for the determination of the ratio of the radii is similar to 
that outlined by Callendar, and is a development of Bosscha’s method. A thick 
brass tube carrying a delicately suspended series of magnets and a mirror mounted at 
its centre was rigidly attached to the framework of the large coils. The small coils 
could be mounted co-axially on this tube against accurately turned shoulders at a 
distance of about 30 cm. on either side of the centre of the fixed coils (see Plate 2). 
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