CLARK CELL AS A. STANDARD OF ELECTROMOTIVE FORCE. 
603 
§21. Cells set up in accordance with the Second Form of the Specification. 
Of tlie cells described in Table VIII., Nos. 71, 72, 73 were made on May 30. The 
paste was prepared according to Lord Rayleigh’s instructions, already quoted, from 
mercurous sulphate, supplied by Hopkin and Williams. The zinc rod was supplied 
by Harringtons, and is stated to contain a trace of iron. Before using the paste was 
shaken up with mercury. The mercury used in 72 and 73 was supplied by Hopkin 
and Williams as “ pure redistilled.” 
No. 71 is Muirhead’s portable form ; the platinum wire is bent into a spiral, and 
dipped while red-hot into mercury. There is no free mercury at the bottom of the 
test tube. 
Nos. 74, 75, 76 were made on June 3, and Nos. 77, 78, 79 on June 11, following 
the directions of the second form of the Board of Trade memorandum, which was 
issued June 23 (see p. 584). Mercury distilled in the laboratory was used; the zincs 
were from Harringtons, and the mercurous sulphate from Hopkin and Williams. 
The zinc sulphate, marked “Brit, Pharm.,” from Hopkin and Williams, was used in 
all the cells. The cells Nos. 71-76 were taken to the Board of Trade towards the 
end of June, and brought back to Cambridge on July 18. Nos. 77, 78, 79 were lent 
to Mr. Griffiths on August 14. Details of their further history are given in 
Table IX. 
It will be seen that the above cells, with the exception of No. 76, agreed with the 
standard to within one in a thousand from the beginning. 
No. 73 was somewhat low during three tests in June, resembling in this respect 
the cells already put up ; in all the other cases the agreement is very close. 
Two comparisons with the Board of Trade standards are given at the end of the 
Table. These will be discussed later. The cells were brought back to Cambridge on 
July 20. Two other cells, Nos. 86, 87, were made on July 20 in the same way, 
using mercurous sulphate from Bchuckharts. They were both within one of the 
standard. 
Table IX. gives the tests on the cells lent to Mr. Griffiths. He made careful 
daily comparisons, using No. 43 as a standard. The results of two of his observations 
are given in the first two columns; the others were quite consistent. Nos. 69 and 70 
were also treated by him as standards. The other four, Nos. 65, 77, 78, 79, were 
allowed to produce current through a resistance of 20,000 ohms. 
On September 13 the E.M.F. of No. 43 fell suddenly by nearly two per cent., and 
gradually recovered, regaining its normal value on September 17. On September L8 
No. 70 was too high, and continued so till September 23, when the tests ceased. 
On September 21 and 23 No. 43 was too low. 
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