VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
305 
coil; and, what is also of some importance, it admits of repetition at pleasure with 
comparatively little trouble. 
§ 20. The sensitiveness of this arrangement was about the same as in Series II., and 
the table shows a good agreement among the results obtained on different days. The 
final number from this series is ‘9868, almost the same as from Series I. and II. 
The small difference of effective resistances required for balance in the two positions 
of the induction coils, amounting to about one part per thousand, is almost exactly 
accounted for by the small difference of distances of mean planes in the two cases, as 
deduced from Professor Chrystal’s measurements of the thicknesses of the flanges. 
In the first position (see § 24) the coils are nearer together by almost exactly one part 
per thousand, a difference which, according to the formula given above (§ 18), should be 
reproduced almost without change in M and therefore in It, the greater values of 
M and It corresponding to the smaller distance. 
§ 21. If we combine all the results of the present investigation, giving equal weights 
to the two arrangements of the induction coils, we have 
1 B.A. unit = \98677 X 10 9 C.G.S. 
With use of the ratio between the mercury unit and the B.A. unit found by us 
(Proc. Boy. Soc., May, 1882), this gives 
1 mercury unit=’94150 X 10 9 C.G.S. ; 
or, which is the same thing, the ohm is the resistance of a column of mercury at 
0° centigrade whose section is 1 square millim., and whose length is 
1062T4 millims. 
We now pass on to the details of the measurements. 
Details of Measurements. 
Diameter of disc. 
§ 22. Preliminary measurements of the disc while still mounted were made on 
August 11, 1882, with callipers by Messrs. Elliott. Bead by the vernier of the 
instrument itself the mean diameter was 
2ct = 310’76 millims. 
The opening of the callipers was also determined independently by reference with 
the aid of microscopes to a verified scale of millimetres. In this way 
2a=310’77 millims. 
The circumference was also measured by a steel tape, afterwards compared with 
the millimetre scale. Correction being made for the thickness of the tape, the 
j result was 
2c« = 310’84 millims. 
MDCCCLXXXIII, 2 R 
