A STANDARD OF ELECTRO-MOTIVE FORCE. 
797 
scarcely have been expected, in view of the very small quantities of zinc which they 
contain. 
If we diminish still further the proportion of zinc in the weaker amalgam, the E.M.F. 
increases ; until, if we omit the zinc altogether, the E.M.F. may reach ‘8 or ’9 Clark. 
In this case, however, the E.M.F. is very unsteady. 
On Feb. 18, 1885, a cell (F) of the H-pattern was charged with pure mercury in one 
leg, and in the other with an amalgam containing one milligram of zinc dissolved in 
30 grams of mercury. No paste being used, the cell was filled up with saturated zinc 
sulphate solution. Under these conditions the E.M.F. was unsteady—about - 7. On 
Feb. 23 some paste was added to the leg containing pure mercury, on which the 
E.M.F. became steady at ‘939 Clark, even when the amalgam was stirred with a 
glass rod. On Feb. 28, E.M.F. = ’935 ; on March 2, ‘931 ; on March G, '920 ; on 
April 12, '068 ; and on Dec. 2, '0003. 
Another cell (G) charged with pure mercury and paste in one leg had no zinc 
added to the pure mercury in the other leg, except such as it received by the 
passage for 3 m of the current from two large Clarks, in whose circuit 1000 ohms 
of wire-resistance was also included Under these circumstances, the deposited zinc 
should have been about mg. The resulting E.M.F. was '923, falling off from day 
to day, as in the case of (F). Nine months afterwards the value was '0003. 
From these and other experiments it appeared that an astonishingly minute 
proportion of zinc in the amalgam was sufficient to give the cell most of the force 
of a Clark. I had contemplated examining in greater detail the electromotive 
character of weak amalgams of known composition, when my attention was called 
by Prof. Ayrton to an interesting investigation by Messrs. Hockin and Taylor,* 
covering the same ground. In their cells zinc was opposed to the various amalgams ; 
so that the circumstances were really the same as in the case of H (1, -^), H (1, q), 
above discussed, the strong amalgam being equivalent to zinc. A few of their 
numbers may here be quoted. When the zinc was opposed to pure mercury, 
E.M.F. = IT86 volt. (According to my experience this should have been uncertain.) 
The mercury being replaced by amalgams, the E.M.F. observed were as follows :— 
Volts. 
Zinc 1 part, mercury 23,600,000 parts . . . IT79 
„ „ „ 11,800,000 „ ... 1'080 
7,530,000 „ ... -655 
„ „ „ 5,900,000 „ ... -513 
1,800,000 „ ... -214 
400,000 „ ... -134 
„ „ „ 200,000 „ ... -124. 
* “On the Electromotive Force of Mercury Alloys.” ‘Journal of Society of Telegraph Engineers,’ 
vol. viii., 1879. 
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