CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
599 
full detail the same information as to the changes which occur immediately after 
setting up. These do not occur in the cells 50, 51, and 53, at least to so marked an 
extent, but the effect is shown in 52. 
Now, Nos. 50 and 51 are made of the same materials but crystals of zinc sulphate 
were added to 50, and the zinc rod was pushed a good deal further down so as to be 
well immersed in the paste which settles above the mercury. This may produce an 
important difference. 
Nos. 57 and 58 were made simultaneously on April 25. The liquid used was 
purposely not warmed at all, and had been standing for some time in the room. It 
was saturated at the temperature of the room. The two cells agree in having a 
smaller temperature coefficient than the standard, but they differ in E.M.F. There 
is probably some slight difference in the temperatures at which the two are saturated. 
The results are only of interest as emphasising the importance often insisted on by 
others of introducing crystals of zinc sulphate, and making sure that they are in 
excess at all the temperatures at which the cell will be used. 
In cells 59 and 60 the treatment was the same, except that in 59 the zinc rod was 
washed as usual with dilute sulphuric acid, and in 60 this was omitted. No. 60 has 
agreed with the standard throughout, in No. 59 there was a sudden and unexplained 
drop in E.M.F. between May 21 and August 7. Of the above cells, Nos. 57 and 58 
are not Clark cells as defined in the specification. The others, with the exception 
of No. 59, attained their normal value within a month of being set up, and have 
retained it since. 
§ 19. Cells of Modified Pattern set up by L.R.W. and S.S. in March , 1891. 
The next set of cells were put up in the form indicated in the Board of Trade 
specification, p. 601, though at some time previous to its issue. The test tubes used, 
however, were rather larger, being about 10 cm. high and 2 to 2'5 in diameter. The 
platinum wires making contact with the mercury are enclosed in glass tubes, and 
were heated to a red heat before being placed in the mercury, so that proper contact 
was ensured. 
Nos. 36 to 41 were put up by Mr. Wilberforce on March 18. In the first three 
the platinum wire is sealed into the glass tube which encloses it, in the last three 
this has not been done. On April 21 Nos. 36, 37, 40, and 41 were much too low, and 
it was noticed that there was a thick grey deposit on the zincs, the zincs from these 
cells and from No. 38 were withdrawn and scraped, they were then replaced and 
again tested, the results are somewhat too high, but from that time on, with the 
exception of No. 41, they have been fairly consistent. Nos. 61, 62, 63 were cells of 
the same pattern, made on March 18, by one of us (S.S.), at the same time as No. 56 
and of the same materials, while No. 64 was made in the same way as No. 60 on 
Apri] 24. 
