THE EARTH’S ^MAGNETIC FIELD IN INTERNATIONAL UNITS. 
441 
wire no attempt was made to smooth down any slight roughness produced hy the 
shellac varnish having stiffened projecting filaments of the silk covering of the wire. 
Before the second set, however, such roughness was removed hy lightly rubbing the 
surface of the coil with a smooth piece of brass. Hence the fact that the first 
measurements gave a larger value for the radius is not to he wondered at. 
The radius below the first layer was in the same way measured by the two 
methods. The individual measurements of the diameters are given in the following 
table :— 
Diameters at Bottom of Groove. 
Station. 
Coil A. 
Means. 
Coil B. 
Means. 
I. 
11 . 
I. 
H. 
1-7 
59-960 
59-962 
59-961 
59-962 
59-961 
59-961 
2-8 
59-962 
59-961 
59-962 
59-960 
59-962 
59-961 
3-9 
59-962 
59-962 
59-962 
59-962 
59-962 
59-962 
4-10 
59-963 
59-962 
59-962 
59-960 
59-962 
59-961 
5-11 
59-961 
59-960 
59-961 
59-960 
59-960 
59-960 
6-12 
59-960 
59-961 
59-960 
59-961 
59-962 
59-962 
Means . 
59-961 
1 _ 
_ 
59-961 
Radius. . . 
29-981 
' - 
— 
29-981 
Correction for temperature of scale . 
+ •008 
— 
+ •008 
Radius at 16^ 
i 
29-989 
— 
— 
29-989 
Radius ot Coils at Bottom of Groove at 1G°. 
Method. 
Coil A. 
Coil B. 
From circumference (1). 
29-991 
29-990 
„ „ (2) . 
29-991 
29-991 
,, diameters (1). 
29-989 
29-989 
„ „ (2) . 
29-989 
29-989 
Means . . . '. . 
29-990 
29-990 
The mean radius of a coil is only ec^ual to the mean of the external and internal 
radii if the distribution of the wire throughout the cross-section of the coil is uniform. 
To test this point, measurements made during the winding of the coil are useless, as 
the winding on of the upper layers is likely to compress the lower layers. If, however, 
as in the present case, the wire has been soaked in shellac, so that the turns are 
bound together, the measurements taken when unwinding the coil are not subject to 
VOL, CXCVIII,—A. 3 L 
