286 Prof. Fran/ Exner on Contact Electricity. 



metal in connexion with the zinc is also oxidizable in air, still 

 the theory evidently remains the same ; the difference of 

 potential of two metals is always measured by half the dif- 

 ference of their heats of combustion. 



Thus, in order to prove the truth of the chemical theory, one 

 has only to compare the observed differences of potential with 

 the heats of combustion so exactly determined by J. Thomsen. 



Such a comparison shows at once a general correspondence 

 between experiment and calculation, as also concerning the 

 absolute value of the differences of potential. The figures, 

 however, that have been given by different authors for the 

 tension-differences (on account of reasons which we shall 

 presently have opportunity to examine) agree only in a 

 moderate degree. I have therefore endeavoured to obtain 

 with all possible exactitude the difference of potential for at 

 least a few metals. For this purpose I adopted exclusively 

 the method of Kohlrausch. Thomson's method with the 

 ring electrometer, though in principle the simplest, could not 

 be used, because in the later experiments it was requisite to 

 have the metals in different gases. 



A condenser was accordingly formed from the metal to be 

 examined and a thick plate of platinum, the latter insulated 

 by paraffin from its metallic support. The metals examined 

 were Zn, Fe, Cu, Ag, all in well-ground plates of 55 millims. 

 diameter, as also the platinum plate. The insulating film of 

 the condenser was of paraffin; and metal handles were fastened 

 to the plates with the same material. The two poles of an 

 insulated standard Daniell were now connected with the con- 

 denser-plates alternately in one sense and the opposite. The 

 charges of electricity produced are proportionate to the sum 

 and difference of the electromotive force of the plates and the 

 Daniell, the ratio of which can therefore be ascertained from 

 these two observations. As a check experiment the condenser 

 was directly connected, and the resulting charge determined. 

 The measurement of these quantities was made by a Branly's 

 quadrant electrometer, the quadrants of which were charged 

 by a Zamboni's pile. Its deflections were, according to a 

 previous graduation within the prescribed limits, proportionate 

 to the charges. 



The first experiment related to the tension between pure 

 zinc and platinum. I must remark that for the attainment 

 of constant results it is absolutely necessary to clean and dry 

 the plates well before every experiment, and also to work in 

 a warm room. Also before every experiment the paraffin layer 

 must be carefully tested, and, if it be electric, renewed by 

 remelting. This, however, cannot of course be done between 



