136 Dr. J. A. Fleming on the Use of DcmielVs Cell 



vided always that no interdiffusion of solutions has taken 

 place, and that the zinc retains a perfectly bright untarnished 

 surface. 



4. Influence of Temperature on the Electromotive Force of the 

 Normal Daniell Cell. 



The researches of experimenters who have studied the Clark 

 cell have established the fact that its E.M.F. diminishes with 

 rise of temperature. The exact value of the coefficient of 

 variation seems to depend on the mode in which the cell is 

 made up ; and very careful examination has been made of the 

 Daniell cell as above described, to see if its real E.M.F. is 

 affected by temperature. Some writers have asserted that it 

 is ; but it is obvious that the variation of internal resistance 

 must be eliminated entirely by the use of a null method of 

 observation of the E.M.F. The condenser method is not a 

 sufficiently accurate one to apply, and the potentiometer 

 method is the most trustworthy. 



Two U-tube cells were prepared with 1*4 sp. gr. zinc sul- 

 phate and 1*1 sp. gr. copper sulphate ; and these cells were 

 respectively immersed, the one in a water-bath at 20° C. and 

 the other in melting ice at 0°. A pair of zinc and copper 

 poles was likewise prepared ; and when the solutions in the 

 U-tubes had acquired the temperature of the baths, the E.M.F. 

 of each cell was taken with the pair of zinc and copper plates, 

 first in one and then in the other, with the following results : — 



Plates in the warm cell at 20° C. Plates in the cold cell at 0° C. 

 E.M.F. of cell. E.M.F. of cell. 



1-081 1-082 



1-079 1-082 



1-079 1-082 



1-078 1-083 



The experiments showed a very small but decided fall of 

 E.M.F. as the cell is warmed, and at the rate of about 3 parts 

 in 1000 for 20° C. This is only about ^ of the variation of 

 a Clark cell for the same range ; and, practically, we may say 

 that the E.M.F. of a Daniell cell is independent of tempera- 

 ture for such range as occurs in the natural temperature of 

 the air in our climate *. This quality of the normal Daniell 



* The same result has been obtained by Mr. Preece in his experiments 

 on the effects of temperature on the electromotive force and resistance of 

 batteries, see ' Electrician/ March 3, 1883, vol. x. page 367. He gives a 

 table showing the E.M.F. at various temperatures, obtained by the con- 

 denser method, and shows that there is no perceptible change in E.M.F. 

 of a normal Daniell between 0° and 17°, and a slight fall subsequently of 

 about 9 parts in 1000 between 17° and 28°, but a rise, however, after 

 reach ins* (33°. 



