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PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 31 
Oertling’s works with satisfactory results ; a difference of one-tenth of a 
milligramme may be detected. 
2. The marble cylinders and fittings.—Insulation and permeability 
tests were made on various samples of marble early in the year; even- 
tually First Statuary Carrara Marble was chosen as most suitable for the 
work. An experimental marble cylinder was wound with a double helix 
and the insulation satisfactorily carried out; the results of the tests 
leave little doubt as to the advantages of the double helix. The winding 
of both suspended cylinders has now been completed, and it is anticipated 
that the fixed cylinders will be finished in September. The linear 
measurements and insulation tests have yet to be made. Unless unfore- 
seen difficulties arise the balance equipment should be completed, and 
the whole ready for preliminary observations by the end of the year. 
During the early part of the year Mr. F. E. Smith completed his 
researches into the construction of a mercury unit of resistance, of 
which some account was given in the last report. The results have been 
communicated to the Royal Society and are being published in the 
‘Philosophical Transactions.’ The values of the various tubes (eleven in 
number) are very accordant, and a mercury standard of resistance of 
a high degree of accuracy now exists. Since the completion of his work 
the specification of the Clark cell has engaged Mr. Smith’s attention, and 
a detailed account of his work forms an Appendix to the present Report. 
Mr. Smith has amply confirmed the result of previous investigators that 
the greater part of the difficulty in obtaining entirely concordant results 
for the various cells set up by different experimenters is due to the 
mercurous sulphate. He describes three methods of preparing the paste 
which lead to identical results, and which have the advantage that cells 
set up with these pastes have, the same E.M.F. within one or two hundred 
thousandths of a volt immediately after manufacture. In the first 
method due to Professor Divers and Mr. Shimidzu the paste is prepared 
by the action of fuming sulphuric acid on mercury ; in the second, follow- 
ing Professor Carhart, it is prepared by the electrolysis of weak sulphuric 
acid and mercury ; while in the third mercurous sulphate is dissolved 
over a water bath in sulphuric acid. The acid solution is then poured 
into a large volume of distilled water and the mercurous sulphate is 
precipitated in a pure form. In all cases it is important that, as advised 
by Mr. Swinburne and Professor Carhart, the salt should be washed, for 
a Clark cell, with zinc sulphate, and for a cadmium cell with cadmium 
sulphate, and not with distilled water. Mr. Smith is continuing his 
inquiries and hopes shortly to be able to issue a complete specification 
for Clark and cadmium cells. The completion of the Ampére Balance 
will enable an absolute determination of their E.M.F. to be made. 
The Committee regret to report that no further progress has been 
made since their last report with the experiments to determine the per- 
manence and reliability of the platinum resistance thermometers de- 
scribed in that report. 
It was pointed out last year that a special resistance box was required 
to enable the work to continue ; unfortunately the funds necessary for 
its purchase were not forthcoming, and the work has remained stationary 
for a year. 
The Committee would consider it most unfortunate if work of 
& very real importance, on which a start has already been made and 
