440 
PROFESSOR W. WATSON ON A DETERMINATION OF THE VALUE OF 
turned so that it had a concave curvature equal to that of the coil as deduced from 
the tape measurements. These small curved gauge-pieces were held against the 
outside layer of the vure by clamps, and their internal curved edges formed a sharp 
line to which the cross wires of the microscopes could he adjusted, these microscopes 
looking through the holes in the flanges. Two independent sets of measui’ements 
v^ere made for each coil, and the results are given in the following table :— 
Diameters outside Layer 8. 
1 
Coil A. 
Coil B. 
Station. 
I. 
II. 
I. 
11 . 
1- 7 
60-670 
60-670 
60-677 
60-679 
2- 8 
60-670 
60-671 
60-678 
60-680 
3- 9 
60-662 
60-660 
60-689 
60-688 
4-10 
60-657 
60-658 
60-684 
60-680 
5-11 
60-648 
60-647 
60-666 
60-667 
6-12 
60-664 
60-664 
60-666 
60-669 
Means. 
60-662 
60-662 
60-677 
60-677 
1 Radius. 
30-331 
30-331 
30-339 
30-339 
Correction for temperature 
of scale 
I 
+ -009 
+ -009 
+ -009 
+ -009 
1 
Eadius at 16°. 
1 
30-340 
30-340 
30-348 
1 
30-348 
i 
It thus appears that the maximum departure from a perfect circle amounts to 
0‘014 centim. in coil A and to O'OII centim. in coil B. That is, to about two parts 
in ten thousand. 
The results for the external radius obtained by the two methods are collected in 
the following table :— 
Badius of Coils outside Layer 8 at 16°. 
Method. 
Coil A. 
Coil B. 
From circumference (1) . . . 
30-346 
30-348 
„ „ (2) . . . 
30-342 
30-349 
,, diameters (1) .... 
30-340 
30-348 
„ (2) . . . . 
30-340 
30-348 
Means. 
30-342 
30-348 
Of the two values obtained by the tape method, the first measurement was made 
before the magnetic measurements, and for fear of damaging the insulation of the 
