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



metal as the other plate, the E.M.F. actually set up by the 

 tendency to produce this ultimate change varies with the 

 nature of the second metal; and whilst the E.M.F. produced 

 is rarely, if ever, precisely equal to 7i%J even at first, when a 

 current has been made to circulate for some short time the 

 E.M.F. is found to have diminished to an amount depending 

 on the rate of evolution of hydrogen on the surface of the 

 other plate, the character of its surface, the temperature, &c. 

 This is usually expressed by saying that the plates become 

 "polarized," or that a "counter electromotive force" x is 

 set up, diminishing the effective E.M.F. of the arrangement 

 from E, which it was at first, toE— x, the value of x varying 

 with circumstances. 



From the principle of the conservation of energy it results 

 that, when the current circulates, the difference between the 

 work h%J x Q which would be performed during the passage 

 of a quantity of electricity Q sufficient to decompose a gramme- 

 equivalent of substance, and (E— «#)Q the work which the 

 current can actually perform, must make its appearance as 

 sensible heat. If, therefore, E — x < h% J, the electromotor must 

 rise in temperature from this cause — irrespective of the rise 

 due to the fact that, as the current traverses the electromotive 

 equally with the rest of the circuit, heat is developed therein 

 in proportion to its resistance in accordance with Joule's law 

 (§ 1). If, however, E— #>/^J, the battery is of necessity 

 proportionately cooled ; and if the experiment is so carried 

 out that the resistance of the battery is small compared with 

 that of the rest of the circuit, so that the heat developed in 

 the battery in accordance with Joule's law is but small, the 

 cooling effect may be made to preponderate over the heating 

 effect, so that an absorption of heat on the whole takes place 

 in the battery. 



15. This effect has been experimentally studied by Favre 

 (Comptes Rendus, lxvi. p. 252, lxvii. p. 1012, lxviii. p. 1300, 

 and lxix. p. 34), who has shown that under the last-named con- 

 ditions an absorption of heat takes place during the solution 

 of zinc or cadmium in hydrochloric acid, whilst heat-evolution 

 takes place if sulphuric acid be substituted for hydrochloric. 

 In other words, the secondary actions set up, as a whole, 

 absorb heat in the first case and evolve it in the second. 

 Similarly in two-fluid batteries, such as Daniell's and Grove's, 

 analogous results were obtained, although in these cases " po- 

 larization " by evolution of hydrogen could not possiblv be 

 produced. With cells after Daniell's construction heat was 

 found to be evolved by the secondary actions, though not to a 

 very large extent (about 4' 3 per cent, of the heat due to the 



