386 
MR. E. H. GRIFFITHS ON THE VALUE OF 
which was freshly distilled, was about 1 centim. in depth, and was nearly touched by 
the ends of the zinc rods. About 1 centim. of the “ paste ” was placed on the 
mercury, then, about 1'5 centim. of ZnSO^, solution and some crystals of ZnSO^. 
Above this solution was fixed a thin cork, on which was a thin layer of parafiin wax. 
On this was placed about 6 centiras. of glass-wool and then about 2 centims. of marine 
glue. The diameter of the cell was about 2‘5 centims. The platinum wire was 
sealed into a thin glass tube, the end of which projected above the marine glue. In 
most cases the upper end of the zincs was completely burled. 
Nos.* 31 to 42. — HggSO^t unwashed but shaken up with Hg and the ZnSO^ solution 
filtered at 30° C., zincs not amalgamated. 
Nos. 43 to 48.—Prepared by following out the Board of Trade directions exactly. 
Nos. 49 to 54.—Boiled the ZnSO^ solution with ZnO, allowed to cool to 30° C. 
Filtered at 30° C. Added to this a small quantity of Hg^SO^ 
paste. Filtered off resulting black precipitate, warmed filtrate, 
and again filtered at 30° C. The cells made with this solution 
appeared to settle down to their final E.M.F. immediately after 
their manufacture, unlike the others, which were some time 
before they became steady. 
About thirty comparisons of these cells were made between January and September, 
1892. Nos. 31 to 42 remained at the Cavendish Laboratory for some time, and were 
repeatedly compared with the Bayleigh Standard by Mr. Skinner, to whom we owe 
our best thanks for the trouble he took in this matter. 
We select as examples comparisons made on three different dates. The results 
of the comparisons on other dates are as close, but it is unnecessary to give them 
at length. 
No discrepancy showed itself except after rapid changes of temperature, and in the 
most extreme case the difference from the mean did not amount to 1 in 2000. The 
comparison of cells 31 to 36 with the Cavendish Standard, on February 13th and 
20th, gave precisely similar results. These comparisons were only the last of a 
series. On both dates the temperature of the standard was slightly higher than 
that of the cells, and therefore the equality was very close. Mr. Skinner, writing 
on February 20th, said, “ You may take it that Nos. 31 to 36 are now equal to our 
standard, and no further comparison is necessary.” In our comparisons the cells 
were, in each case, placed in opposition to No. 31. 
* These cells are numbered 131 to 160 in jMessrs. Glazebkook and Skinner’s paper, ‘ Phil. Trans., 
A, 1892, pp. 622 to 624. Numbers 154 to 160 were not used bj us during this investigation, 
t Chemicals supplied by Messrs. Harrington. 
