140 Daniell' s Cell as a Standard of Electromotive Force. 



these methods have been employed with success for some time 

 in standardizing the working galvanometers used in the lamp 

 factory of the Edison and Swan United Electric Light 

 Company. 



In conclusion may be given other results, collected from 

 various authors, respecting the electromotive force of the 

 Daniell-cell. 



Wiedemann (Galvanismus, vol. i. cap. iv., Bestimmung der 

 Electromotorischen Kraft, p. 341) gives a description of 

 Raoultfs cell (see Baoult, Ann. de Chim. et Phys. [4] t. ii. 

 p. 345, 1864), and states that Raoult finds that copper-foil has 

 higher electromotive force than electro-deposited copper by 

 about 200 5 & n d attributes it to the oxides of copper contained 

 in it. He also confirms the invariability of E.M.F. with tem- 

 perature at least between 10° and 50°; and states that with 

 pure amalgamated zinc and electro-deposited copper, solution 

 of saturated sulphate of copper and solution of sulphate of 

 zinc, containing 1 part of crystals to 1 part of water =1*35 sp. 

 gr. at 20°, the electromotive force is 1*124 electromagnetic 

 units. This, corrected by the Cambridge value of the B.A. 

 unit, is 1*109 volt. 



Sir W. Thomson (p. 245 of ' Papers on Electricity and 

 Magnetism 9 ) finds the electrostatic measurements of a 

 DanielPs cell to be '00374. The nature of the solutions and 

 electrodes are not given. Taking the value of v as 3 x 10 10 , we 

 have the E.M.F. as 1*12 B.A. volt, which, reduced to true 

 volts, gives 1*105 volt. 



Lord Rayleigh gives the ratio of a Daniell set up with 

 amalgamated pure zinc, electro-deposited copper, and solu- 

 tions of sulphate of zinc and copper, each of sp. gr. 1*1. 

 Five observations of the ratio of this cell to that of a known 

 Latimer-Clark cell at 16° C, taken as unity, gave *7702, 

 •7710, -7705, -7698, -7702, mean -7703 ; and since the Clark 

 is 1*435 volt, this gives the Daniell as 1*105 volt. 



Latimer Clark (Journ. Soc. Tel. Eng., January 1873) gives 

 it as 1*11, which, reduced to true volts, is 1*095; and Dr. Alder 

 Wright (Proc. Phys. Soc. London, vol. vi. p. 292) gives the 

 value of a normal Daniell set up with solutions of the same 

 molecular strength, preferably of strength m M S0 4 100 OH 2 , 

 where m is near 2 — that is with copper sulphate nearly satu- 

 rated, and zinc sulphate of equivalent molecular strength, and 

 pure amalgamated zinc plates and electro-deposited copper — 

 as 1*114 B.A. unit, or 1*099 true volt. 



None of these are very far from the value assigned to the 

 standard cell described above, viz. 1*102 volt, with equidense 

 solutions and metals as described. 



