708 
LORD RAYLEIGH OH THE CLARK CELL AS 
“ Mercury alloyed with any number of metals takes the place in the scale of E.M.F. 
of the most electro-positive metal it may contain, if the amount of the electro-positive 
metal present is not less than about yooo o?o i n weight of the mercury.” 
§ 55. In order to obtain any stability of E.M.F. in these cells, it seems to be 
necessary that the mercury should either be alloyed with a sensible, though perhaps 
relatively very small, portion of zinc, or else protected with mercurous sulphate. 
"With pure mercury without paste not only is the E.M.F. variable from hour to hour, 
but it can scarcely be said to be definite even at a particular time—that is, it may 
be altered by the passage of a very small quantity of electricity, such as should 
have no effect whatever upon a properly constituted cell. Sometimes when the 
galvanometer contact was made, a throw of the needle was observed, not followed 
by any corresponding permanent deflection, the cell in fact behaving like a charged 
condenser. 
In the absence of paste a very small addition of zinc to mercury gives it a 
definite and tolerably permanent character. If, however, there be any mercurous 
sulphate, even though originally in another leg of an H-cell, the mercury is gradually 
repurified. To this cause is to be attributed the gradual fall to zero of the E.M.F. 
of the cells (F) and (G) above considered. 
Indeed, the whole tendency of these observations is to suggest that the action of 
the mercurous sulphate may be secondary, rather than primary; so that if zinc could 
be opposed to really pure mercury, no mercurous sulphate would be needed in a Clark 
cell It may be, however, that in that case a minute quantity of mercurous sulphate 
would form itself spontaneously. In such cells as H (1, -|) we are compelled to 
suppose that the chemical origin of the E.M.F. is the tendency of zinc and mercury 
to combine, or rather the tendency of two different amalgams to equalize themselves. 
There is here a close parallelism to the electromotive forces due to affinity of saline 
solutions of different strengths, manifested, for example, when an electrode of zinc is 
in contact with a strong solution of zinc sulphate, and the second similar electrode 
is in contact with a weak solution. 
Before quitting this subject, I may mention observations on what was practically 
a Clark cell, although jnepared without zinc and without paste. On March 7, 1885, 
an H-cell (J) was charged with pure mercury in both legs, and filled up with zinc 
sulphate solution. A current from an external source of about yrtj ampere was then 
passed through it for 3^ hours. The E.M.F. at subsequent dates were as follows :— 
Clakk. 
March 9.083 
March 11 '980 
April 9. - 978 
July 7.973 
December 12.071 
