360 Dr. C. R. A. Wright on the Determination of 



the manganese dioxide) ; but by-and-by this ceases and the 

 E.M.F. goes down. This effect is often said, vaguely and not 

 strictly correctly, to be due to the plates becoming " polarized " 

 by hydrogen, the which phrase really applies to the wholly 

 different phenomenon now under consideration. 



That the E.M.F. of a Daniell cell is somewhat less when 

 generating a current than when no current passes has already 

 been shown to be the case by TValtenhofen (Silzb. Akad. Wien, 

 xlix. 2, p. 229) and by Beetz* using a modification of the method 

 of compensation (Pogg. Ann. cxlii. p. 581,1871). Apparently, 

 however, physicists generally are under the impression that 

 the diminution in E.M.F., or "polarization " of the cell, thus 

 produced is practically inappreciable : thus, for instance, 

 within the last few months a determination of the value of v 

 has been made in Sir W. Thomson's laboratory by R. Shida, 

 by measuring the E.M.F. of a gravity Daniell, first, in elec- 

 trostatic units (no current being generated), and then in elec- 

 tromagnetic units (a moderately powerful current being set 

 up) ; the which method evidently can only be applied if the 

 difference in the E.M.F. according as a current is generated 

 or not is negligible (Brit. Assoc. Reports, 1880, p. 497). The 

 same kind of remark applies to several other investigations 

 published during the last few years. 



96. Decomposition of Dilute Sulphuric Acid with a Zinc 

 Positive Electrode. — On trying experiments with a zinc (amal- 

 gamated) positive electrode and a platinum electrode in the 

 same way as those described in§ 92, it was found that the nume- 

 rical value of e was uniformly negative and decreased as the cur- 

 rent increased. So long as the current remained of a constant 

 value the nature and dimensions of the external battery were 

 immaterial ; so that the same value for e was obtained ivhether 

 an external battery was actually applied, or ivhether the zinc- 

 platinum couple ivas the sole electromotor in circuit, provided 

 that, by suitably adjusting the total resistance in circuit, the 

 current was made the same. Thus the following average 

 numbers were obtained with a cell containing dilute sulphuric 

 acid of 10 grammes H 2 S0 4 per 100 cubic centims., the plates 

 each exposing 8*6 square centims. of surface, and being con- 

 tained in a U-voltameter like those above described, of 1*1 

 square centim. bore, the centres of the plates being 13 cen- 

 tims. apart ; so that, as the specific resistance of sulphuric 

 acid of the strength used is close to 2*9 ohms at 18° (from 

 Kohlrausch's numbers, loc. cit. suprci), the value of R was 



13x2*9 OJ . 

 close to — r— — = 64: ohms : — 



