CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
605 
Table IX. 
Date. 
Sept. 12. 
Sept. 17. 
Nov. 11. 
Nov. 23. 
Nov. 25. 
Dec. 2. 
Dec. 7. 
Tempera¬ 
ture. 
14-6 
14-4 
11-1 
13-2 
12-8 
14-4 
15-5 
65 
0 
0 
0 
— 4 
0 
0 
-2 
69 
1 
1 
-6 
-2 
-1 
-1 
-3 
70 
-3 
-3 
— 2 
-3 
-2 
0 
-1 
77 
-1 
-4 
3 
— 2 
1 
-1 
-1 
78 
-1 
-1 
0 
-5 
-3 
-1 
-3 
79 
-1 
-1 
-1 
-4 
-2 
-1 
-3 
43 
S 
S 
1 
-1 
1 
1 
0 
The cells were returned at the beginning of November. The tests given in 
Table IX. show that they have retained their E.M.F. with practically no change. 
Thus we may say that all the cells set up by us after the last date of the Board of 
Trade circular agree among themselves and with our standard to about one part in 
three thousand. 
§ 22. Errors Arising at the Zinc Pole. Effect of Amalgamation of the Zinc. 
After a cell has been made, the paste, which, when poured into the test tube is 
fairly homogeneous, and of the consistency of cream, separates into two portions. The 
more solid part, the mercurous sulphate, settles to the bottom, resting on the mercury, 
and a clear liquid remains on the top. The zinc rods used varied somewhat in length ; 
in some cells they were entirely in the clear liquid, in others they extended partly 
into the more solid portion below. Various observations had shown us that this 
might produce some difference in the behaviour of the ceil. 
The differences appear to arise from the fact that when the zinc rod touches the 
mercurous sulphate it becomes properly amalgamated. A small trace of iron in the 
zinc, or in the zinc sulphate solution, does not, then, produce any serious effect. If, 
however, the zinc is only in the clear liquid, the amalgamation does not always take 
place. The zinc, instead, frequently becomes covered with a grey deposit. This grey 
deposit consists in the main of mercury in very fine globules, and when it is formed 
all over the zinc the E.M.F. of the cell falls, and it becomes unsteady. 
The experiments recorded in Table X. show the effects of this deposit and of the 
method of treating it. 
The cells in question were six, Nos. 42-47, constructed towards the end of April by 
Mr. L. It. Wilberforce. The materials of the cells are indicated as above, by Greek 
letters. They are of the Board of Trade pattern, but rather larger, being put up in 
test tubes about 10 cm. high and 2'5 cm. in diameter. It will be seen from the Table 
that while on April 27 they were nearly right, on May 2 they had most of them fallen 
considerably, and by May 26 they were, on the average, some 35 units too low. It 
