CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
(Ml 
Date. 
January 23. 
January 30. 
February 6. 
March 11. 
Number. 
5 
•9777 
•9587 
•9633 
1-0005 
6 
•9823 
•9590 
•9658 
1-0072 
7 
•9848 
•9627 
•9629 
1-0159 
8 
•9969 
•9516 
•9626 
•9943 
9 
•9892 
•9632 
•9711 
10 
•9695 
•9459 
Writing in May, 1891, .Dr. Schuster says :— 
“ My present opinion is that the cell only arrives at its steady state after some 
action has taken place between the zinc and the solution, and it is necessary that the 
zinc surface exposed should not be too small compared with the quantity of liquid. 
At any rate, most of my large cells prepared with good sulphate seem to settle down 
at an E.M.F. about ^ per cent, too low, and the lower the smaller the surface of zinc 
exposed. Amalgamation seems a mistake, for it is just the action of the zinc previous 
to amalgamation which brings the cell into a steady state.” 
Writing at a later date, February, 1892, Dr. Schuster expresses the opinion that 
the action required to bring the cell to a steady state is the formation of a basic zinc 
sulphate. This process may be facilitated by the introduction of zinc oxide into the 
cell, and in that case amalgamation will probably be an advantage, but if the cell is 
left to settle down of itself, the process of amalgamation helps the formation of the 
basic zinc sulphate, and it is desirable, therefore, that this process should go on 
in the cell. 
He also tells us that in the cells made in the Bnichsanstalt at Berlin the zinc 
sulphate is boiled with zinc to secure the formation of the basic salt. 
On this point, however, our own view would be somewhat different from that 
expressed by Dr. Schuster, for it has been observed by one of us (S.S.), and by 
Mr. E. H. Griffiths, and is in accordance with the chemical theory of the interchange 
of bases, that the mercurous sulphate which is used in preparing the paste reacts with 
the basic zinc sulphate, producing zinc sulphate and probably basic mercurous sulphate ; 
thus, if the zinc oxide used to neutralize the zinc sulphate does form with it a basic 
salt, the mercurous sulphate clears this out of the solution. 
Dr. Schuster has also confirmed our results as to the danger of the grey deposit 
and the rise in E.M. F. produced by removing it. 
Mr. Swinburne, in his paper already quoted, also recommends amalgamation. He 
found differences of potential amounting to '4 per cent, of the E.M.F. of a Clark cell 
between different specimens of zinc immersed in saturated zinc sulphate. These he 
assigns partly to “ impurities, probably iron, in the zinc sulphate.” They disappeared, 
practically entirely, when the zincs were amalgamated. 
It may be noted that Lord Bayleigh did not meet with this source of error, for his 
4 i 2 
