32 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
with what appears to be a fair measure of success, and the corrections due to changes 
in the coils can in certain cases be calculated. 
It follows, therefore, that if the B.A. unit of any particular period is known in 
terms of the resistance of a column of mercury, and if the coils used for the absolute 
measurements remained constant in resistance from the time of their measurements 
in terms of a mercury column to the time of their absolute measurement, the results 
given in Table I. can in all cases be reduced to give the length of the column of 
mercury having a resistance of 1 ohm. 
But it is very probable that the resistance of many of the coils did not keep 
constant, and it is not possible for us to reduce the results except in a few cases. 
The instances referred to are the determinations by Lord Bayleigh, by Dr. Glaze- 
brook, by Viriamu Jones, and by Ayrton and Jones. In all of these cases 
comparisons were made with the B.A. standard coils, and the details of these 
comparisons have been preserved and published. In the Report of the B.A. Electrical 
Standards Committee for 1908, the changes in resistance of the coils used by Lord 
Rayleigh and Dr. Glazebrook have been traced, and a comparison of the mercury 
standards of resistance made by Lord Rayleigh, Dr. Glazebrook, and F. E. Smith 
is given in Table VIII. of the same report. This comparison, together with the notes 
on the standards used, enables us to express Lord Rayleigh’s and Dr. Glazebrook’s 
results in terms of the present mercury standards of the National Physical Laboratory. 
This we have done, the results being marked (S), while (A) indicates the results given 
by the author. 
Referring first to Lord Rayleigh’s determination in 1882, we find that 
comparisons were made with certain B.A. unit coils and with mercury standards of 
resistance. However, the terminals of the latter were not at 0° C. but between 5° C. 
and 6° C., and it was shown by Dr. Glazebrook in 1888 that an error of 24 parts 
in 100,000 was introduced because of this. If we apply a correction of this 
amount, Lord Rayleigh’s 1882 value of the ohm in centimetres of mercury becomes 
106'24 (l+0'00024) = 106'26 5 , and the 1883 value becomes 106'214 (l +0'00024) 
= 106'239. These values are given in Table I. (within 1 part in 10,000) as 106'26 
and 106'24. 
Dr. Glazebrook’s determination of the ohm was made in 1882, and he constructed 
mercury standards of resistance in 1888. The principal resistance coil employed in 
1882 was a platinum-silver coil known as “ flat,” and this also was used in 1888. In 
the interval it was assumed to have kept constant—there was at that time no certain 
evidence to the contrary. A careful survey of the history of the coils, which is 
published in the B.A. Report for 1908, shows, however, that “flat” increased in 
resistance in the interval 1882—1888 by 41 parts in 100,000. Dr. Glazebrook’s 
value for the ohm in centimetres of mercury is 106'29, and this, when corrected for 
the change in the resistance coils, becomes 106'29 (l —0'0004l) = 106'25. 
The coils used by Viriamu Jones in 1894 were compared with the B.A. standards 
