170 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
Substituting these values in expression (7) and solving for a, we get 
a = 5-96076 cm. at 20° C., 
2 g x = 2(7rN 2 a 2 + 1 L ? 7rN 2 ^ 2 ) = 88521-8 at 20° C. 
and hence 
V. The Calculation of the Dynamometer Constants. 
We have to evaluate 
(- 4G ,g l - 4G 5 g 5 P / 5 - 4G 7 gr 7 P' 7 ). 
The following data are given, elsewhere, in this paper:— 
At 20° C. :— 
The mean radius “ a ” of the large coils = 
„ distance “cl” of the large coils = 
,, channel depth f, of the large coils : 
The number of turns N x on each large coil 
Hence we find by substitution that 
26. = - = 13*7268 at 20” C. 
24‘8905 cm. 
12-4448 „ 
1‘72 
38. 
a 
127r'N 1 a 2 [d i -%a 2 d 2 +~ 
2G 5 =-A——ny-^ = + 0-000008242 at 20° C. 
5 (a 2 + d 2 ) k 
2G 7 = - l^rN i CT 8 (df-^tfdt+^cdd 2 -^) = __ q-0000000104 at 20° C. 
(a 2 + d 2 ) h 
We have also from §IY. (f) 
2 g x = 8852D8 at 20° C., 
and since 
a = 5-96076, § = 2‘9917 and N 2 = 396, 
we have at 20° C. 
and 
2 g 5 = 10-N 2 a 2 (V-f a 2 ^ 2 +| 4 j = -106000000, 
2 g 7 = U7r^ 2 n 2 (^-^oL 2 S i + ^a. i S 2 --^od) = +4830000000. 
If f) — 7° 55' 29T" (see § VII. (i)) then <f> = 82° 4' 30-9" and therefore P' 5 = +1"3902 
and P' 7 = —1"14. 
