250 Dr. C. R. A. Wright on the Determination of 



cible by the sum of the chemical and physical actions taking 

 place in an electromotor and that corresponding to the current 

 actually obtainable from the electromotor, Raoult calls the first 

 the chemical heat of the pile, and the latter the voltaic heat 

 thereof ; whilst the terms chemical heat and galvanic heat have 

 been employed by Edlund to indicate these quantities respec- 

 tively (Phil. Mag. 1877, vol. iii. pp. 428 & 501); so that the 

 above results may be indicated by saying that the chemical 

 heat of a Daniell battery is 26,122 gramme-degrees (per 

 gramme-equivalent of zinc dissolved), whilst the voltaic or 

 galvanic heat is 23,900, and so on. Several points touched 

 upon in these papers are of interest as bearing on the problem 

 of the determination of the amount of E.M.F. corresponding 

 to the primary effects of electrolysis ; their discussion, like 

 that of several analogous memoirs by previous observers, is 

 postponed till a later occasion. 



18. In order to apply the general method above indicated 

 (§11) for the determination of the E.M.F. corresponding to 

 the sum of the chemical and physical actions taking place du- 

 ring electrolysis, it is requisite to determine: — first, the differ- 

 ence of potential subsisting between the electrodes ; secondly, 

 the quantity of electricity passing in a given time (known 

 from the weight of substance decomposed); and, thirdly, the 

 heat evolved as such in the electrolytic cell in that time. The 

 first quantity may be found approximately by the aid of a sine 

 galvanometer of very high resistance. A method which is in 

 principle substantially this has already been used by Raoult 

 (loc. cit.); this will be more fully discussed further on 

 (§ 23). 



Another way of determining it may be based upon obser- 

 vations with a voltameter, thus : — The amount of decomposi- 

 tion produced in a given time t by passing a current from the 

 battery employed (of E.M.F. = E') through the voltameter 

 is noticed ~n x ; the electrolytic cell is then placed also in cir- 

 cuit, and the amount of decomposition in the voltameter in the 

 same time t again determined = w 2 , and also the heat pro- 

 duced in the cell = h. R being the resistance of the battery, 

 voltameter, and connexions, and r being the resistance of a 

 wire " obstructing the current equally with the electrolytic 

 cell," the currents flowing in the two cases are respectively 



C l —n 1 xk=-^ r and C 2 = w 2 x^ = -fs > 



±i sx + r 



where k is a constant — whence 



R _ ^ and * = %~ % . 

 R + r V R + r n x 



