THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



NOVEMBER 1889. 



XLIII. On the Clark Cell as a Source of small Constant 

 Currents. By Richard Threlfall, M.A., Professor of 

 Physics , University of Sydney, and Arthur Pollock, B.Sc* 

 [Plates XII. & XIII.] 



THERE are several methods by which the potential-differ- 

 ence (for the future written P.D.) of the plates of a 

 voltaic cell may be measured. To measure the E.M.F. it is, 

 as a rule, necessary to prevent the cell under examination from 

 yielding any appreciable current while the estimation of its 

 terminal P.D. is being made. 



The electrometer is consequently the most obvious instru- 

 ment to use in such an investigation ; but, as a matter of fact, 

 it is infinitely inferior both in convenience and accuracy to 

 the galvanometer. 



All the experiments about to be described were made with 

 a galvanometer in the manner adopted by Lord Rayleigh and 

 Mrs. Sedgwick (Phil. Trans, ii. 1884, " On the Electrochemical 

 Equivalent of Silver, and on the Absolute Electromotive 

 Force of Clark Cells "). The method admits of an estimation 

 of the terminal P.D. of a cell, either at rest or when (by the 

 use of separate sets of resistance-coils to short-circuit its ter- 

 minals) it is yielding a current of any desired value. 



_ Let E be the E.M.F. of the cell, P.D. its terminal potential- 

 difference, C the current passing through it, R its internal 

 resistance, R' the external resistance short-circuiting its ter- 

 minal, A the observed change of P.D. at any instant when the 



* Communicated by the Physical Society : read March 23, 1889. 

 Phil. Mag. S. 5. Vol. 28. No. 174. Nov. 1889. 2 D 



