130 Dr. J. A. Fleming on the Use o/DanielVs Cell 



experiments concerning temperature, the whole U-tube can 

 be immersed in a vessel of water or ice up to nearly the top 

 of the reservoirs, and the temperature in the solutions taken 

 by means of a thermometer passing through the rubber-cork. 

 Each of the electrodes can be removed for examination or 

 change without in the least disturbing the surface of contact 

 of the solutions. If experiments are being made in which 

 the snlpliatc-of-copper solution is the denser, the position of 

 the solutions is interchanged. The bottle II serves to collect 

 the waste solutions. 



The electrodes are made of rods of the purest zinc and 

 copper, about 4 inches long and J inch diameter. The zinc 

 found most suitable is made from zinc twice distilled and cast 

 into rods ; the copper prepared by electro-depositing on a 

 very fine copper wire, until a cylinder of the required thick- 

 ness is obtained. 



With these appliances to hand, all the facts recorded by 

 previous experimenters have been carefully repeated and con- 

 firmed, and the influence of each variable upon the electro- 

 motive force examined. The results are collected as follows. 

 For the sake of brevity, a Daniell's cell in which zinc, sulphate 

 of zinc, sulphate of copper, and copper are the elements will 

 be called a normal Daniell ; and the statements here below 

 refer to the variations in the difference of potential between 

 the poles of the normal Daniell, when sending either no 

 current or only an infinitesimal one, caused by variations in 

 tho physical state of the four elements. In each set of 

 experiments the greatest care was taken to keep all the 

 elements constant, except the one which was purposely being 

 varied in order to detect the influence of it on tho wholo 

 electromotive force. 



1. The Effect of Variation in the Copper Surface and 



Condition. 



In these experiments the copper- and zinc-sulphate solu- 

 tions were sometimes of the same specific gravity, sometimes 

 different; and in some cases the zinc electrode was a rod of 

 tho purest cast zinc amalgamated with pure mercury, some- 

 times a rod of commercial zinc wire cleaned but not amalga- 

 mated. Some scores of observations were made with identical 

 zinc plate and solutions, but various kinds of copper plates*; 



* All tho following values are given in real volts, taking Lord Rayleigh's 

 value for the moan 1>.A. 11111101' resistance as equal to 'VS(u earth-quadrant 

 per second. 



Tho determinations have hwn made against a Clark cell, kindly com- 

 pared by Lord Raylelgh with his No. 1 cell, whoso value is given in his 



