474 



KEPOItT — 1884. 



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

 less length of a graduated wire convoying a current ; and the position of 

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

 and testing until no el:argo at all is found. The step of potential on the 

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

 this would have caused a charge in a similar but uncom])ensated con- 

 (.lenser, 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 K.IM.F. of 

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

 ring cxperiniem, thus makin<j: it very analogous to that of Pellat.' 



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 T-epresents 

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

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

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

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

 coats (/m couches rlcctriiiid.i)^ 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 tho numbers being 

 given in volts : — 



' UirrEREXCE OI" rOTKXTl.M, OF TlfK Ki.ECTKIC CO.VrS WHICH COVKK A MkTAI. 

 AND STASOAni) GOLO METALLICALLY CONNECTED AND IIOTH IN All!.' 



' I tind indeed that Sir W. Thomson eoniplctely niitieijiatcd Pellat in t lie appli- 

 cation of Lralvanic compensation to the measiirenient of Volta elfects ; for in .yiitun- 

 April 14, 1H.S1 (vol. xxiii.]). 5(i7), is printed an account, 1,'iven in brief at theSwansc;! 

 meetinf; of the Uritisli Association (sec Trans, of Srcfx., 1880, p. 4i)4), which relatis 

 how the divided ring: cxperimeit naturally developed into more comi)lete couipoi:- 

 nation with slide resistances, and that an extensive series of measurements weiv 

 made on this ])lan in the years ISo'.t-Gl with results (piitc in agreement with tiiosf 

 published by Hankel in 18(12. Other experimi>iits were made since 1801 with ri'siilt- 

 confirmatory of those of I'faff, 1 8L'l , .showing the Volta elfect to be independent of tin 

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

 something new should be obtained by the method, and was not published untiU'ellaU 



