THE EARTH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
439 
Distance between the Mean Planes of the Coils. 
Station. 
Distance between mean planes. 
Width of groove. 
I. 
H. 
HI. 
Mean. 
Coil A. 
Coil B. 
1 
30-299 
30-295 
30-296 
.30-297 
0-597 
0-590 
2 
30-296 
30-297 
30-300 
30-298 
0-593 
0-591 
3 
.30-299 
30-298 
30-298 
30-298 
0-592 
0-589 
4 
30-301 
30-299 
30-299 
30-300 
0-593 
0-594 
5 
30-300 
30-299 
30-298 
30-299 
0-590 
0-593 
G 
30-299 
30-299 
30-296 
30-298 
0-592 
0-592 
7 
30-303 
30-302 
30-302 
30-302 
0-593 
0-592 
8 
30-299 
30-298 
30-303 
30-300 
0-590 
0-593 
9 
30-300 
.30-298 
30-302 
30-300 
0-592 
0-592 
10 
.30-.301 
30-301 
30-303 
30-302 
0-592 
0-593 
11 
30-301 
30-299 
30-,302 
30-301 
0-591 
0-591 
12 
30-301 
30-299 
30-300 
.30-300 
0-592 
0-591 
Mean 
30-2996 
0-592 
0-592 
Correction for temperature of scale . . 
+ -0084 
Mean distance at 16° C. . 
30-3080 
' 
To obtain the mean radii of the coils, the radii lielow the first layer and over the 
eighth layer were measured. These measurements were made in two ways. In the 
first method a piece of steel clock spring of such a width that it would fit into tlie 
groove was reduced at either end to half its widtli. A fine fiducial line was ruled at 
one end, and a scale, each division of whicli was ‘02 inch, was ruled at the other end 
])y means of a dividing machine. Prolongations of the jiart of the spring used in 
the measurements were left at either end, by holding which the spring could be 
wrapped tightly round the groove. The values of the divisions of the scale, as well 
as the distance between the fiducial line and the zero of the scale, were determined 
in terms of the standard metre, a second line being ruled half-way along the strip 
for this purpose, so that each half was less than a metre. In this way the length of 
the circumference w^as measured, and the radius to be measured was taken as the 
radius calculated from this circumference, less half the thickness of the steel tape. 
As a check on the measurements made with the tape, and also to see whether the 
coils were truly circular, the diameters of the coils were measured directly in six 
directions. For this purjiose, after the magnetic measurements were complete, 
twelve small oval holes were milled through the outer part of the flanges so as to 
expose the wire and also the bottom of the groove. A very light cut was taken, and 
only continued just down to the surface of the wire, which was not in any way 
displaced. In order to give a sharp edge, to which the cross wire of the microscope 
might be set when reading the outside diameter of the coil, two small brass plates, 
about 8 centims. long and of such a thickness that they would just fit into the 
grooves, were prepared. One edge of each of these plates was then very carefully 
