14 Mr. E. F. Herroun on the Electromotive 



two Daniell cells or a Fuller's bichromate. This precaution, 

 however, was usually found to be superfluous, as the circuit 

 of the third battery was only closed when an observation was 

 being made, and its E.M.F. and internal resistance varied 

 only within very small limits. 



In order to ascertain if the E.M.F.'s measured were really 

 due to the essential chemical changes occurring within the 

 cell, it was usual, after making an observation with an ex- 

 perimental cell when first put up, to allow the cell to work 

 for a short time on a circuit of a few ohms, so as to cause 

 the surfaces of the metals to become dissolved and deposited 

 upon respectively. The circuit was then opened, and after a 

 short rest the E.M.F. was again determined. If this differed 

 by little or nothing from the previous measurements it was 

 considered to be the true value for that cell ; if, on the con- 

 trary, there was an appreciable difference further measure- 

 ments were made. By operating in this way it was found 

 that copper plates (particularly electro-deposited metal) which 

 had been exposed to the air, and had acquired a superficial 

 film of suboxide, usually gave, when first put up, an E.M.F. 

 somewhat in excess of their true value. After allowing the 

 cell to send a current for a few minutes, the E.M.F. fell to 

 the normal value, and thereafter remained sensibly constant. 

 ( Vide also Dr. Fleming, " Use of DanielPs Cell/' Proc. Phys. 

 Soc. vol. vii. p. 168.) 



As such initial fictitious electromotive forces would affect 

 to their full extent the results obtained by a method like that 

 of Poggendorf or Latimer Clark, or in fact any method in 

 which difference of potential and not the current is measured, it 

 was thought well to control such measurements with one or 

 two cells by using Wheatstone's method of adding a resist- 

 ance to bring down the deflection of a galvanometer-needle 

 through a given number of degrees. The results obtained 

 by this means agreed remarkably with those found by the 

 previously mentioned process. 



In the calculations relating to the heat of formation of 

 some of the salts, it has been taken as a mean value that the 

 electrochemical equivalent of hydrogen is '0001038*, and 

 that the Joule equivalent is 4*16 x 10 7 ergs. From these 

 numbers the factor which, multiplied into the heat, expressed 

 in gramme-degrees, evolved by the displacement of the 

 gramme equivalent of one metal by that of another, will give 

 the electromotive force in volts, is found to be 4*318 x 10 -5 

 or 4*32 x 10" 5 nearly. 



* u On the Determination of Chemical Affinity in terms of Electromo- 

 tive Force,'' by Messrs. Wright and Thompson, Phil. Mag. vol. xix. p. 7. 



