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XVII. On the Electro-chemical Equivalent of Silver, and on the Absolute Electromotive 
Force of Clark Cells. 
By Lord Rayleigh, D.C.L., F.R.S., and Mrs. H. Sidgwick. 
Received June 18,—Read June 19, 1884. 
[Plate 17 .] 
§ 1. In former communications* to the Royal Society we have investigated the 
absolute unit of electrical resistance, and have expressed it in terms of the B.A. unit 
and of a column of mercury at 0° of known dimensions. The complete solution of 
the problem of absolute electrical measurement involves, however, a second determi¬ 
nation, similar in kind, but quite independent of the first. In addition to resistance, 
we require to know some other electrical quantity, such as current or electromotive 
force. So far as we are aware, all the methods employed for this purpose define, in 
the first instance, an electrical current; but as a current cannot, like a resistance, be 
embodied in any material standard for future use, the result of the measurement must 
be recorded in terms of some effect. Thus, several observers have determined the 
quantity of silver deposited, or the quantity of water decomposed, by the passage of 
a known current for a known time. In this case the definition relates not so much to 
electric current as to electric quantity. A more direct definition of the unit current, 
and one which may perhaps be of practical service for the measurement of strong 
currents of 50 amperes or more, would be in terms of the rotation of the plane of 
polarisation of sodium light, which traverses a long column of bisulphide of carbon 
enveloped by the current a given number of times.! 
Other observers have expressed their results as a measurement of the electromotive 
force of a standard galvanic cell. In this case it is necessary to assume a knowledge 
of resistances. The known current in passing a known resistance gives rise to a 
known electromotive force, which is compared with that of the cell. 
In the present communication are detailed the experiments that we have made to 
determine the electro-chemical equivalent of silver, and the electromotive force of 
standard Clark cells. As regards the choice of silver there is not much room for a 
difference of opinion. The difficulties to be overcome in the use of a water volta-' 
meter are much greater. Copper is, indeed, employed in ordinary laboratory practice 
* Proc., Ap. 12, 1881; Phil. Trans., 1882, Part II.; and 1883, Part I. 
t See Camb. Phil. Proc., Nov. 26, 1883. 
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