On the Electromotive Force of certain Tin Cells. 13 



data for the calculation of a correction. But at any rate it 

 seems misleading to exhibit the result as determined by the 

 average number of turns per unit length, when it really de- 

 pends also upon the ratios of the rates of winding at the various 

 parts of the length. 



Eepeating that I throw out these remarks merely for pur- 

 poses of discussion, I remain, 



Your obedient Servant, 



Ratleigh. 



III. A Note on the Electromotive Force of certain Tin Cells. 

 By E. F. Hereoun, of King's College, London* 



HAYING failed to find any published account giving the 

 electromotive force of cells consisting of tin in solutions 

 of any of its salts, opposed to other metals in solutions of their 

 corresponding salts, I thought that some measurements re- 

 cently made by myself might be of interest to this Society 

 as tending to complete the series of double-fluid cells already 

 investigated and recorded by others. 



The cells examined consisted of an outer containing-vessel 

 of glass or glazed earthenware, in which was placed one of 

 the metals in a solution of one of its salts, and an inner porous 

 pot 8 centim. high containing the other metal in a solution 

 of its corresponding salt. It will be seen that they were, 

 therefore, similar in construction to the ordinary laboratory 

 form of Daniell cell, and the standard employed in these ex- 

 periments was a Daniell cell made up in exactly the same 

 manner. 



The solutions in the Daniell cell were copper and zinc 

 sulphates of equal molecular strength as recommended by 

 Dr. C. Alder Wright, the proportion being 1-8 M"S0 4 , 100 

 H 2 0. The cell made up in this way was assumed to have an 

 electromotive force of 1'08 volt, and it probably differed but 

 little from this value. 



Method of Measurement. 



The measurements of the E.M.F. of the cells were made by 

 the zero method of Latimer Clark, in which no polarization is 

 possible when the adjustment is effected. The standard Daniell 

 cell was used before and after each experimental cell in order 

 to avoid error due to variation in the potential of the wire, 

 resulting from changes in the E.M.F. or internal resistance 

 of the third or polarizing battery, which consisted either of 



* Communicated by the Physical Society : read November 14, 1885. 



