VALUE OE THE BRITISH ASSOCIATION UNIT OE RESISTANCE. 
307 
A. 
B. 
Rim (marked side) .... 
•478 
•446 
Channel. 
1-896 
1-899 
Rim (unmarked side) 
•488 
•465 
Total thickness of rim . 
2-862 
2 810 
Now that the rings are wound it is difficult to verify these numbers. However, the 
total thickness of the rings at the places touched by the distance pieces in the 
arrangement used for Series III. was taken, with the result 
A. 
B. 
Mean of three places.... 
2-8625 
2-8067 
These latter values of the thicknesses will be used in the calculation of Series III. 
In Series I. and II. the rings were not reversed, and we must use the numbers above 
given for the thicknesses of rims which were contiguous to the slips of glass ; but in 
this case the result is not at all sensitive to changes in the distance of mean planes. 
The rims contiguous to the glass were for both coils the marked rims, of which the 
aggregate thickness is ’924. If we add to this the thickness of the glass strips *454, 
we obtain 1‘378 as the distance between the wire sections. Again, adding the mean 
a^xial width of section 1'897, we find as the distance of the mean planes 
26 = 3'275 centims. 
Fv<j. 4- 
WIRE 
The distance-pieces . 
§ 24. The measurement of the distance-pieces used for the third series was made 
with great care. As only the mean is required, the three pieces were held under the 
microscopes in one length by a nut and a long bolt running through. Readings were 
taken in several positions, as the pieces were turned round, and reference was finally 
2 E 2 
