80 The Clark Cell a Standard of Electromotive Force. [Jan. 21, 



fluctuations of temperature. Under favourable circumstances two 

 cells will retain their relative values to 100 oo for weeks or months 

 together. 



Unless carefully sealed up, the cells lose liquid by exudation and 

 evaporation, and then the electromotive force gradually falls. Marine 

 glue appears to afford a better protection than paraffine-wax, and 

 there seems to be no reason why cells thus secured should not remain 

 in good order for several years. 



In cells of the H-construction (§ 29 of former paper) the leg con- 

 taining the amalgam (but not the one containing pure mercury) is 

 liable to burst, apparently in consequence of a tendency to alloy with 

 the platinum. Protection with cement of the part of the platinum 

 next the glass has been tried, but no decisive judgment as to the 

 adequacy of this plan can as yet be given. 



Recent cells, intended for solid zincs, have been made of a simplified 

 pattern — nothing more, in fact, than a small tube with a platinum 

 wire sealed through its closed end. The zincs are not recast, and the 

 paste is prepared from (unwashed) mercurous sulphate rubbed up in 

 a mortar with saturated solution of zinc sulphate and a little zinc 

 carbonate. A stock of paste may be prepared and retained for use in 

 a bottle. 



Experiments are described tending to prove that the irregularities 

 observed during the first few weeks of the life of a cell prepared with 

 acid materials, have their origin principally at the mercury electrode. 



Cells prepared with dilute solutions have a lower temperature 

 coefficient (about 0*00038), but would be more difficult to use as 

 standards whose value is to be inferred from the mode of preparation. 



Details are given of H-cells charged with amalgams of zinc and 

 mercury in both legs, without mercurous sulphate. A very small 

 proportion of zinc is sufficient to produce the maximum effect. Pure 

 mercury, neither alloyed with zinc nor in contact with mercurous 

 sulphate, has an uncertain electromotive value. 



Since the comparison of cells does not absolutely exclude a small 

 general alteration of electromotive force with age, further determina- 

 tions of the standard cell (No. 1) have been effected by means of the 

 silver voltameter. The results — 



Table XVIII. 



Date. 



E.M.F. of No. 1 at 15° C. 

 in B.A. volts. 



October, 1883, to April, 1884 



November, 1884 



August, 1885 



1 -4542 

 1 -4540 

 14537 



are very satisfactory, and indicate a constancy sufficient for almost all 

 practical purposes. 



