CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. G13 
these cells, as the observations show, but the error produced will be small. The 
change per 1° relative to our standard is about 2'8 of our units. Our standard 
changes by about 4 - 5 units, so that the change in the Griffiths cells is only about 
4'5 — 2'8 or 17, and this implies a coefficient of about one-third that of the standard 
cell. The last column gives the values found at the Board of Trade in terms of 
their standards. 
This, and the cells described in Table VII. are, so far as our experience has gone, 
the only sets of cells which have been quite wrong and yet have remained consistent 
among themselves for some time, and that over a considerable range of temperature. 
Table XIII. 
Date. 
1 
August, 1888. 
December, 1888. 
August, 1890. 
August 8, 1891. 
August 10, 1891. 
November 14, 1891. 
December 16, 1891. 
March 9, 1891* 
Temperature .... 
15 
15 
15 
16-8 
17-4 
11T 
16-2 
11-5 
Number. 
91 
26 
27 
26 
26 
29 
12 
23 
12 
92 
28 
29 
28 
28 
31 
15 
26 
15 
93 
24 
24 
24 
25 
27 
10 
22 
11 
94 
28 
28 
26 
26 
28 
12 
24 
12 
95 
28 
28 
26 
27 
29 
12 
24 
13 
96 
28 
28 
28 
28 
31 
14 
25 
14 
§ 25. Comparison with the Board of Trade Standard Cells. 
A means of comparing the E.M.F. of our cells with those of the standards of the 
Board of Trade at Westminster, which were set up by Dr. Muirhead, is afforded by 
the last columns of Tables II., VIII,, and XIII., which give, in terms of the unit we 
have adopted, viz., '00025 volt, the differences between our cells and the standards of 
the Board of Trade. The comparisons were kindly made for us by Mr. Bennie, 
Major Gardew’s assistant. It will be seen at once that the agreement is very close. 
For the cells Nos. 7, 8, 9, 10, Table II., constructed by Mr. Callendar, the differ¬ 
ences observed are rather greater than those found by us either before the cells were 
sent or soon after they returned. 
The last two columns of Table VIII. give the results for the live cells Nos. 71-76 
No. 71 appears to have been low at one of these observations, for the rest we see 
that the differences are rather greater than those with our own cells. 
* Comparison with the Board of Trade standards. 
