THE 

 LONDON, EDINBURQH, am> DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOUBNAL OF SCIENCE. 



-♦ 



[FIFTH SERIES.] 



JANUARY 1878. 



I. On a Form of Daniell Cell convenient as a Standard of Elec- 

 tromotive Force. By Oliver J. Lodge, D.Sc* 



[Plate L] 



ALTHOUGH a volt is the formal unit of electromotive 

 force, yet it happens in practice that differences of po- 

 tential get stated as equal to so many Daniell cells more fre- 

 quently than any thing else, showing that there is some decided 

 convenience in this mode of statement, a convenience partly 

 owing, no doubt, to the fact that a freshly set-up Daniell is a 

 tolerably uniform and easily reproduced standard. An ordi- 

 nary Daniell, however, is by no means suitable as a standard, 

 because of the diffusion of the copper-liquid through the porous 

 cell. This defect must obtain in any cell where two liquids 

 separated by a pof ous partition are employed ; and hence 

 attempts have been made to construct standard cells with solid 

 electrolytes, or with mercury instead of copper salts, as in the 

 little cell devised by Mr. Latimer Clark, which, though not 

 absolutely constant, is still, I suppose, the best for its special 

 purpose. But all cells with solid electrolytes are extremely 

 inconstant, in the sense that they suffer greatly from short- 

 circuiting and take some time to recover themselves; and 

 there are some other inconveniences attending the use of a 

 large number of Clark's cells. 



A convenient Daniell Cell with high internal Resistance. 



Of all known cells, a Daniell charged with sulphate of zinc 

 and sulphate of copper seems to be the most perfect — in this 



* Communicated by the Physical Society — one of the cells haying- 

 been exhibited at a meeting of the Society in February 1877. 



Phil. Mag. S. 5. Vol. 5. No. 28. Jan. 1878. B 



