[ 213 ] 



XXIV. On the Electromotive Force of Voltaic Cells having an 

 Aluminium Plate as one Electrode. By A. P. Laurie, 

 B.A., B.Sc* 



IN a former number of this Journal f I have given an ac- 

 count of some experiments undertaken with the view of 

 testing whether the E.M.F, of a cadmium-iodine cell was really 

 due to the combination of cadmium and iodine. As there 

 explained, these experiments were made on account of my 

 feeling an increasing distrust of measurements of E.M.F. 

 made in the usual way with infinitely small currents, — a dis- 

 trust not of the measurements themselves, but of our right 

 to assume that the E.M.F. of a particular cell was due to a 

 particular chemical reaction. This doubt is obviously justified 

 when once we realize that impurities far out of reach of 

 analytic detection could easily produce sufficient current for 

 the measurement of E.M.F. in the usual way. 



On reading the papers published by Dr. Alder Wright in 

 this Magazine in 1885, 1 was very much struck by the results 

 of his experiments on aluminium-zinc cells, in which he found 

 an E.M.F. actually opposed to that calculated from the thermal 

 data. To quote his figures, given on p. 28, vol. xix. of the Phil. 

 Mag. for 1885, he finds that zinc-aluminium cells, with the 

 metals immersed in sulphate of zinc and potash-alum re- 

 spectively, have an E.M.F. of '538 volt. According to the 

 thermal data, there should be an E.M.F. of *982 volt in the 

 opposite direction, so that we have a difference of 1*519 

 volt between the E.M.F. calculated from thermal data and 

 that obtained by actual measurement. 



Dr. Wright's experiments with other aluminium cells give 

 similar results. 



Now observe that the E.M.F. calculated from the thermal 

 data is obtained by taking the heat of formation of sulphate 

 of zinc and sulphate of alumina respectively, and subtracting 

 the comparatively small heat of formation of sulphate of zinc 

 from the large heat of formation of sulphate of alumina ; that, 

 therefore, the assumption is made that this cell is similar to a 

 Daniell cell, and that sulphate of alumina will be formed and 

 sulphate of zinc decomposed, by the passage of the current. 



The E.M.F. measurement, however, shows an E.M.F. in the 

 opposite direction to that calculated on this assumption. 

 How is this to be accounted for ? Dr. Wright accounts for 

 it by assuming a " thermo-voltaic constant " of 1*519 volt. 

 By this I understand him to mean that he believes that sul- 



* Communicated by the Author, 

 t Phil. Mag. May 1886, p. 409. 



