192 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
Measurements were made at both the north and south ends of the scales where the 
actual deflection readings occurred. In order to insure delicacy of adjustment the 
tube could be supported from the ceiling in the correct position for measurement. A 
sample set is given to show the type of observations taken. 
Table XXII. 
Micrometer readings at 
north ends— 
Micrometer readings at 
south ends— 
Temperature of tube. 
cm. 
cm. 
° C. 
0-313 
— 
18-8 
0-309 
— 
18-8 
0-311 
— 
18-8 
0-314 
— 
18-9 
— 
0-312 
19-0 
— 
0-315 
19-0 
— 
0-315 
19-0 
— 
0-314 
19-0 
Means . . . 0 - 312 
0-314 
18-9 
Mean micrometer reading = 0 313 cm. 
At 20°‘0 C. a reading of 0'300 corresponds to a total length of 298’460 cm., hence 
at 18°'9 C. this becomes 298'454 cm., and a value for a reading of 0'313 is, therefore, 
298'441 cm. 
Each determination of the mean distance between the scales which is recorded in 
the deflection tables was obtained in this manner. 
It is necessary to subtract the correction m for the thickness of the mirror from 
298'450, the standard value for /3. A series of micrometer caliper measurements 
for m gave 0'3584 cm. We have therefore, /3 — m — 298'092 cm. 
(i) Calculation of the Mean tan 6. 
Under the standard conditions enumerated, we have 
hence 
and 
tan 26 
S = 84‘6297 
/3-m ~ 298-092 
0-2839046 ; 
therefore 
26 = 15° 50' 58-14", 
6 = 7° 55' 29-1", 
tan 6 = 0T39201 6 . 
