VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
220 
Position 1 -f ''f ^ Position 2 -f 
l ^ A l \K4 
rtB 
1M 
Position 3 
Position 4 
In the figure It 11 L represents a section of one of the cylindrical rods used to 
separate the coils; its length and the thickness of the walls of the channel are all 
known. If, then, we assume that the median plane of each coil is midway between 
the walls of the channel, we can find the distance between the median planes; this 
we denote by 6. But if the median plane in one coil, B for example, be not halfway 
between the walls but some small distance, 86 suppose, nearer the lower wall, then in 
position 1 the value assumed for b is too large by 86, while in position 2 it is too 
small by 86, and by making observations in positions 1 and 2 and taking their mean 
we eliminate any error in the result which depends on the first power of 86. Simi¬ 
larly, if there be an error in the position of the median plane of the coil A of the same 
nature it is eliminated by inverting A. 
The thicknesses of the walls of the channel as given by Professor Chrystal are:— 
Coil A. 
Coil B. 
centim. 
centim. 
Lettered side.... 
•478 
•446 
Unlettered side . 
•488 
•465 
Also, if 2 h, 2 k be the radial and axial dimensions of the channel for coil A, 2 It, 2k' 
for coil B, we have 
7i=-96 k— ‘95 
li—• 95 k'=’ 95 
Three series of brass cylinders were used to separate the coils, and the lengths of 
these were measured each by two observers. A pair of calipers graduated to read 
with a vernier to part of an inch were found to be the most convenient 
instrument with which to make the measurements. The scale of the calipers was 
tested against the beam-compass without discovering any errors that could affect the 
result to as much as 1 in 10,000. 
The following is the series of measurements for the longest rods used (Series C), 
made by Ik T. G. and given as an example of the agreement of the observations. 
Calling the rods a, /3. y, we found 
a. 
A 
y- 
indies. 
inches. 
inches. 
9-389 
9-388 
9-390 
9-388 
9-389 
9-391 
CO 
oo 
oc 
9-389 
9-390 
