60 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
The curves given in fig. 12 show to what extent the coils vary in diameter. The 
conicality of the coils on cylinders 3 and 4 is much less marked than in the case of 
those on cylinders 1 and 2, but the variation from a circular cross-section is very 
small for all of the coils. The variation of diameter in any cross-section is so small 
2 3 4 5 G 7 Q 9 iO H (2 i 
axial PLANE N° 
Fig. 12. Showing extent to which coils are conical, and variations in cross-section from circle. 
that the mean diameter of the cross-section may be employed for the purposes of the 
calculation of the mutual inductance without appreciable error, but the variation in 
diameter from end to end of the coils necessitates the application of a conical 
correction. 
The probable error of the mean diameter of the coils is estimated to be not greater 
than lju. The probable errors of the gauge bars used are stated on p. 54 ; the 
probable errors of the observations may be estimated from the data given in 
Table III. 
