474 



HEPOET — 1884. 



metals set face to face are connected, not directly, but by a greater or 

 less lengtb of a graduated wire conveying a current ; and the position of 

 the slider on the wire is adjusted by continually separating the plates 

 and testing until no charge at all is found. The step of potential on the 

 wire is then precisely equal to the ' contact force ' between the plates ; for 

 this would have caused a charge in a similar but uncompensated con- 

 denser, and the step of potential on the wire has neutralised it. 



Compensation methods of a sort had been used before by Gerland and 

 by Thomson. Gerland applied compensation to determine the E.M.F. of 

 two metals dipped into a liquid, and Thomson applied it to the divided 

 ring experiment, thus making it very analogous to that of Pellat. 1 



Pellat also adopts Sir William Thomson's view that the Volta effect 

 is due to a true contact force between the metals, and that it represents 

 a real difference of potential between them when in contact ; at the 

 same time he is careful to point out that no rigorous proof can be 

 given of this, and that all that is really and certainly measured electro- 

 statically is the difference of potential between what he calls the electric 

 coats (les couches electrique*), or what may be more simply called the 

 air-films, on the two touching metals. 



The following is a summary of some of Pellat's measurements, gold 

 being the metal with which all are compared, and the numbers being 

 given in volts : — 



' Difference of Potential of the Electric Coats which cover a Metal 

 and Standard Gold Metallically connected and both in Air.' 



1 I find indeed that Sir W. Thomson completely anticipated Pellat in the appli- 

 cation of galvanic compensation to the measur merit of Volta effects ; for in Nature, 

 April 14, 1881 (vol. xxiii. p. 567), is printed an account, given in brief at the Swansea 

 meeting of the British Association (see Trans of Sects., 1880, p. 194), which relates 

 how the divided ring experiment naturally developed into more complete compen- 

 sation with slide resistances, and that an extensive series of measurements were 

 made on this plan in the years 1859-61 with results quite in agreement with those 

 published by Hankel in 1862. Other experiments were made since 1861 with results 

 confirmatory of those of Pfaff, 1821 , showing the Volta effect to be independent of the 

 surrounding gas. The description of all these experiments was therefore withheld till 

 something new should be obtained by the method, and was not publisheduntiirellat's 



