STANDARDS OF ELECTROMOTIVE FORCE. 413 



Unfortunately it is not sufficient to take liquids of the same 

 degree of concentration to obtain a definite electromotive force ; 

 the differences, which no doubt arise from the unequal purity 

 of the metals, may amount to as much as 2 or 3 per cent.* 



Temperature moreover has great influence on the electromotive 

 force of a DanielPs element. According to Von Helmholtz, f 

 this influence is zero when, the solution of copper sulphate being 

 saturated, that of the zinc sulphate is of density i '04 ; the electro- 

 motive force decreases when the temperature increases for the most 

 concentrated solutions of zinc, and increases on the contrary with 

 the temperature for weaker solutions. 



Light seems to have a still more distinct action ; M. Pellat J 

 found that the seat of this action of light is on the copper plate, 

 and that it is due to the more refrangible rays ; it may lower 

 the electromotive force by one or two per cent. 



In Sir W. Thomson's gravity battery, with concentrated solutions 

 of sulphate, the electromotive force is very closely equal to 1*074 

 for a temperature of 15. 



Instead of using this couple in open circuit, Sir W. Thomson |j 

 prefers to take as standards the difference of potential of the 

 two poles, when they are connected by a resistance of 250 ohms. 

 This difference increases at first for some hours after closing 

 the circuit, but it then becomes remarkably constant. 



Latimer Clark's couple consists almost entirely of solid sub- 

 stances, which makes the transport and maintenance more easy, 

 and it seems to give more constant results. The elements of 

 which it is formed are zinc, zinc sulphate, mercuric sulphate, 

 and mercury. 



A concentrated solution of zinc sulphate in boiling water is 

 prepared, and mercuric sulphate is added to the cold solution, 

 so as to form a thick paste, and this mixture is then kept for 

 some time at ioo. The paste is placed on the surface of pure 

 mercury previously heated, and a zinc rod is fixed in it. The 

 electrodes are platinum wires attached to the zinc and to the 

 mercury. From its physical condition, the couple polarizes readily, 



* Lord RAYLEIGH. Phil. Trans. Roy. Soc. for 1884, Part, n., p. 459. 

 t HELMHOLTZ. Berichte der Akad. der Wiss., Berlin, p. 26. 1882. Wiss. 

 Abhand., Vol. II., p. 958. 



I PELLAT. Comptes rendus, Vol. LXXXIX., p. 222. 1879. 



Lord RAYLEIGH. Loc. cit. 



U Sir W. THOMSON. B. A. Report, Southampton. 1882. 



