THE EARTH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 443 
Circumference corresponding to the Axis of the Wire. 
Layer. 
1 
2 
3 
4 
5 
6 
7 
8 
Means. 
Mean of external and internal 
circumference of coils 
Difference. 
Coil A. 
Coil B. 
5-9 
6-1 
11-4 
11.8 
17-1 
17-3 
22-4 
22-8 
27-8 
28-2 ' 
33-0 
34-0 
38-3 
39-4 
43-4 
44-3 ! 
24-9 
25-5 
24-7 
25-2 ' 
0-2 
0*3 
This table shows that, to obtain the true mean radius of the coils, we have to 
increase the mean of the external and internal circumferences in the case of coil A 
by 0'2, and in that of coil B by 0’3. Since one division of the tape corresponds to 
0'015 centim., this corresponds to increasing the mean radius of coil A by 
0‘0016 centim., and that of B by 0’0024 centim. Hence :—• 
1 
Coil A. 
Coil B. 
1 Mean of external and internal radii. 
30T66 
30-169 
Correction for distribution of wire. 
+ •002 
+ •002 
1 Mean radius of coil. 
1 
30-168 
30-171 
The mean radii of the two coils being so nearly alike, we can take the mean of the 
two numbers for the radius of the pair of coils. Thus the mean radius of the coils is 
30T695 centims. at 16°. This value of the radius, as well as the value of the distance 
between the mean planes of the coils given on p. 439, is expressed in terms of the 
brass metre. To reduce these numbers to true centims., we have to deduct 
O'OOOS centim. from the mean radius and 0'0004 centim. from the half distance 
between the mean planes. Thus the dimensions of the coils are as follows :—- 
Mean radius.30’1687 centims. 
Half the distance between mean planes . . . 15T536 ,, 
Number of turns in the two coils.96, 
The coil constant F, which is the field produced at the centre of the coils when 
one ampere is passing, is thus 
F = 1-42671 
at a temperature of 16° C. 
3 L 2 
