190 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
Table XXI. 
Measurement I 
between 
7-5 
and 
92-0. 
7-6 
and 
92-1. 
7-7 
and 
92-2. 
7-8 
and 
92-3. 
7-9 
and 
92-4. 
8-0 
and 
92-5. 
Mean. 
Tempera¬ 
ture of 
scale. 
East scale . . . 
West scale. . . 
84-5042 
84-5001 
84-5036 
84-5002 
84-5015 
84-5000 
84-5034 
84-5007 
84-5028 
84-5003 
84-5039 
84-4999 
84-5032 
84-5002 
°C. 
20-8 
20-6 
Mean .... 
84-5017 
20-7 
The average temperature of the scales during the deflection observations was 
20°'3 G. (see Table XX.). Hence using the temperature coefficient 0‘0000085 for 
milk-glass we see that the mean value for this distance at 20°"3 C. is 84‘5014 cm. and 
a correction of +0'0014 cm. should therefore be added to our deflection mean in order 
to express it in true centimetres. 
(3) Correction for the Mica Windows .—A very small displacement of the reflected 
scale reading is produced by the presence of the thin mica windows through which 
the suspended mirror is viewed. These windows are parallel to the scales and the 
telescope is at right angles to each, hence it can be shown that telescope readings for 
deflection measurements are each decreased by an amount x given by 
x = z (tan 26 — tan f), 
where 2 is the thickness of the mica, 6 is the angle through which the suspended coils 
and mirror are deflected, and <p is the angle of refraction within the mica. In order 
to eliminate this deviation it is therefore necessary to add 2x to the mean value 
for S. 
The mean thickness of the mica windows was 0’0061 cm.; 26 was approximately 
15° 50' and <p can at once be deduced from sin </> = 
sin 26 
where n is the refractive 
index of mica and was taken equal to 1 *58. 
Substitution in the above equation gave x — -|-0‘00066 cm. and hence the desired 
correction 2x = +0’0013 cm. 
(4) Correction Due to the Change in the Controlling Couple .—The last and most 
important correction is that required to reduce the deflection to its initial instan¬ 
taneous value. It was pointed out above that this was equal to 
D-D 
-kt 
1+e 
-k T 
The experimental determination of D and Ic for these wires and for this magnitude 
of deflection is given in the paper on “ Bifilar Suspensions,” quoted before. We have 
