442 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
cannot too strongly emphasise our advice to take too many rather than too few 
precautions. 
When two cells under comparison differ by a considerable fraction, they may be 
compared separately with the Leclanche’s, or rather expressed in terms of the current 
afforded by the Leclanche’s through 10,000 ohms. Thus, on Dec. 3, 1883, in order to 
balance Clark No. 1 (see below) 4926 were required between the points of derivation. 
When a standard Daniell of Raoult’s pattern was substituted for the Clark, the 
number required was 3798. In terms of No. 1 Clark the E.M.F. of the Daniell 
is thus 3798/4926, or '7710. At the end of a series of comparisons it is proper to 
repeat the observation of the first standard cell, in order to check the constancy of 
the current supplied by the Leclanche’s. In our experience there was usually no 
appreciable variation. 
When the cells to be compared are nearly alike, it is better in the second observation 
to express the difference of forces by setting the second ceil to act against the first. 
Thus, the force of Clark No. 1 being expressed as before by 4926, the corresponding 
resistance for the excess of the force of Clark 1 over Clark 3 was 2 ohms. Hence, 
in terms of Clark 1 the force of Clark 3 is '9996, and the result is less liable to 
error than if the comparisons of each with the Leclanche’s were effected separately. 
§ 29. Of the first batch of Clark’s which were compared together from Nov.ember, 
1883, onwards. No. 1 was set up near the beginning, and Nos. 2, 3, 4, 5, towards the 
end of October. They were prepared generally according to the directions given by 
Dr. Alder Wright,'"' to whom we have been indebted for advice and for samples of 
some of the materials. The saturated solution of zinc sulphate was nearly neutral. 
The metallic zinc was bought as pure from Messrs. Hop kin and Williams. The 
mercurous sulphate was from the same source, and the metallic mercury was 
redistilled in the laboratory. We did not consider it desirable to take precautions 
against the presence of air, thinking that it was sure to find an entrance sooner or 
later. 
Four new cells, Nos. 6, 7, 8, 9, were set up from the same materials on January 10, 
1884. It will be seen from the table that when a fortnight old they differed but 
little from the first batch. 
In preparing these cells the most troublesome part of the process was found to be 
the casting of the zincs. The metal, melted in a porcelain crucible, was sucked up 
into a previously heated tube of hard glass, but the operation required some address, 
and there was considerable waste of zinc from oxidation and otherwise. It occurred 
to us to try whether equally, or perhaps still more, satisfactory results might not be 
obtained by substitution for the solid metal of an amalgam of zinc. For this purpose 
a form of cell, called for brevity the IT-cell, was contrived, and is shown full size 
(Plate 17, fig. 5). One of the legs is charged with the amalgam of zinc (B), the other 
with pure mercury (C), covered with a layer of mercurous sulphate (D). The whole is then 
* Phil. Mag., July, 1883. 
