420 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
determination of the mean value of a current of (say) half an hours duration should 
easily be correct to to .ou Tj- 
The fixed coils. 
§ 12. These are the coils of the dynamometer constructed by the Electrical Com¬ 
mittee of the British Association (see § 10). The mean radii of the two coils and 
the dimensions of the sections are very nearly identical, and for our purpose it is 
unnecessary to note anything but the mean. The following are derived from the 
dimensions recorded in Professor Maxwell’s handwriting in the laboratory note¬ 
book :— 
A=mean radiusc=24 - 81016 
2B = distance of mean planes =25 - 00 
2/i= radial dimension of section = 1 *29 
2h =axial.=1‘50 
the unit in each case being the centimetre. 
The number of turns of wire on each coil is 225. 
The above values are those employed in the calculations of the present investigation, 
and they can be only partially verified without unwinding the wire. Owing, however, 
to the final result being comparatively independent of A and B, even a rough 
verification is not without value. The distance parallel to the axis from outside to 
outside of the grooves in which the wire is wound can be found pretty accurately with 
callipers, and was determined to be fO'433 inches. From inside to inside of the 
grooves the corresponding distance was 9’252 inches. The mean of these is the 
distance of mean planes, which is thus 9'8425 inches, or 25'000 centims. exactly. 
This element is, therefore, verified with abundant accuracy. The half difference of 
the two numbers above given represents the axial dimension of the section, and comes 
out P5024 centims., practically identical with 1’50 centims. The mean radius and 
the radial dimension of the section are not now accessible to measurement, but the 
outside circumference agrees sufficiently well with that calculated from the recorded 
dimensions to serve as a verification. 
The number of turns has to be taken entirely upon trust; but the use of the 
method given in Maxwell’s ‘Electricity,’ § 708, makes a mistake in this respect 
very unlikely. Moreover, the electrical comparisons to be detailed later (§ 14) verify 
the equality of the number of windings on the two coils. 
The resistance of each coil is about 14^ B.A. units, and both coils are well insulated 
from the frame on which they are wound. 
