A STANDARD OP ELECTRO-MOTIVE FORCE. 
799 
It will be seen that a tolerable approximation to a Clark was obtained by this 
simple process. Zinc is thrown down upon the mercury in one leg, and mercury 
dissolved in the other by the passage of the current. 
§ 56. The comparisons recorded in Table XIII. furnish abundant proof that there 
has been no special change in the E.M.F. of No. 1, but do not of themselves exclude 
the possibility of a general movement of the cells in one direction as the result of age, 
by which the relative values of cells of nearly tire same date might remain unaffected. 
Comparisons with younger cells, indeed, go a long way towards negativing this 
supposition; but considering that all the older cells had undergone a journey (from 
Cambridge to Terling), I thought that a re-determination of the absolute E.M.F., by 
means of silver, would be a valuable confirmation of their constancy. 
With the assistance of Mrs. Sedgwick, two such silver determinations were effected 
in August, 1885, as described in § 38. Reduced to 15°, the E.M.F. of No. 1 was 
found on August 15 to be F4541 B.A. volts, and on August 19, F4533, giving as a 
mean 1'4537 B.A. volts. The value found from absolute measurements in the autumn 
of 1883, and the spring of 1884, was 1’4542 (§ 36). That given (note to § 37) as the 
result of two experiments in Nov., 1884, is F4534; but a slight error has been 
discovered in the reductions leading to the latter number. By an oversight the factor 
1 - 00041, necessary to correct for the slight heating of the resistance by the passage of 
the current (§ 33), was omitted, the effect of which omission is an under-estimate of 
the difference of potentials at the terminals of the resistance traversed by a known 
current, and consequently of the E.M.F. of the cell under test. This correction being 
introduced, the numbers will stand thus :—- 
Table XVIII. 
Date. 
E.M.F. of No. 1 
at 15° C. in B.A. vol's. 
Oct. 1883 to April 1884 . 
1-4542 
Nov. 1884 . 
1-4540 
Aug. 1885 . 
1-4537 
The slight fall in these numbers has little significance; and we may regard the 
E.M.F. of No. 1 as having remained constant to within about for nearly two 
years. 
I believe that the E.M.F. of cells constructed with reasonable precautions, especially 
as to saturation, will rarely differ at ordinary temperatures by more than joVo P ai ’f 
from 
1-454{1 — -00077(^ — 15)} B.A. volts, 
or (the B.A. unit of resistance being taken equal to ‘9867 ohm) 
1-435(1 —•00077(^ — 15)} true volts. 
5 K 2 
