Messrs. Ayrton and Perry on Contact Electricity. 43 



to point and illustrate his criticism. Friendly contests are at 

 present being waged in the i Educational Times ' among the 

 supporters of rival logical methods ; I hope Prof. Jevons will 

 not take it amiss if I venture to invite him to enter the lists 

 with me, and there make good the charge of " ante-Boolian 

 confusion " which he brings against my method. 

 November 29, 1880. 



VI. Note on Prof. Exner's Papers on Contact Electricity. 

 By W. E. Ayrton and John Perry*. 



I. T~N the autumn of 1879 Prof. Fleeming Jenkin drew 

 J- our attention to a paper by Prof. Exner, read before 

 the Vienna Academy of Science, and appearing in the July 

 number of their ' Transactions ' for that year. This paper will 

 also be found reprinted in this year's April number of Wiede- 

 mann's Annalen der Physik und Chemie ; and quite recently 

 an English translation, prepared by Mr. Brown, has appeared, 

 in the October number of the Philosophical Magazine. 



As then, this paper has been deemed of sufficient import- 

 ance to be printed at least three times ; and as the reasoning 

 employed in it is of so plausible a nature as to mislead a casual 

 reader, and to give him erroneous notions on the subject of 

 contact electricity, we have thought it worth while to draw 

 attention to the inaccuracies it contains. 



The calculation given in the Phil. Mag. for 1851 by Sir 

 Wm. Thomson of the electromotive force of a DanielPs cell, 

 based on the principle of the conservation of energy, is of 

 course well known. The method employed, which was due 

 to Dr. Joule, is as follows : — The work done by a quantity of 

 electricity Q passing between two points at a difference of 

 potential E is EQ. Now if this electromotive force is pro- 

 duced by a Daniell's cell, the preceding quantity of work must 

 be equal to the energy-equivalent of the chemical changes 

 that take place in this cell when a quantity of electriciy 

 Q passes through it. And since this latter can be deter- 

 mnied from the heats of combustion of the products decom- 

 posed and formed, and from a knowledge of Joule's mecha- 

 nical equivalent of heat, Sir Wm. Thomson was enabled to 

 calculate the electromotive force of the Daniell's cell from the 

 supposed known chemical reactions taking place in it. It 

 has been pointed out by Dr. Wright f, and by others, that 

 the great coincidence between the electromotive force of the 



* Communicated by the Physical Society, having been read at the 

 Meeting on November 18, 1880. 



t " On the Determination of Chemical Affinity in Terms of Electromo- 

 tive Force," by C. R. Alder Wright, D.Sc, Phil. Mag. April 1880, p. 247. 



