KLKciuoMOTivE FOi;(i::.> IN iMK voi.TAic ( i:i.r.. 



VJU 



'' 



e true ; 



me '»s 

 U' foici! 

 I'lice or 



u of the 



o assert 

 [•(.ft, lUid 

 iVoct. fifc 

 11' tbiiii:':* 

 .'cuviitely 

 t a juuc- 

 ,'' as I'vo- 

 t is; l)"t 

 has any- 

 its us the 

 ' obsi'ivt'il 

 aiffereuce 

 potontiuls 

 'gt'tl ; ''"' 

 The vial 

 , of the ilif- 

 Yood V<lta 

 i\ whatrwr 



y a coil' 'ft 

 vce at thoii' 



hn sun-oniid- 

 Idel ^ve i^hall 

 Icted vessels, 



Uavby"^";s^ 



' diflerently 

 the vessels. 



Note that tlic dil!'t>renco III" level in this case iinplios no difTcronco of 

 potentiiil, uTid as l)el'i)ri' no work i.^ I'LMiiiiii'il to transfer water hetwoou A 

 !ii.d B J fence it is not easy to distiiiL,'iiisli this case from the former, and 

 tiiis ditficiilty of distintruisliint,' l)etween the two cases is what lias |,'iven 

 rise to nuxst of the contusion. 'I'hi' only easy critoi-ion is the non-oxisteneo 

 in the second case of any Peltier ell'ect at the junction C. Naturally it is 

 jiD^sihlo m1 common for the two ell'ects to bo superposed, hut thi-y are 

 o,»>entia independent. 



Since the two vessels in the second case are at tliesarao potential, the 

 way to observe the effect is to cut and seal the pipe at C, and then show 

 tJMt the vessels are dilTerently char<^ed ; which is what Ndlta did. The 

 uuidel does not indeoil represent the ,t,'railual ciiaiiLfe of polcntiiil induced 

 as the distance lietween the condenser plates increasi;s, and it is scavcoly 

 worth while to complicate the matter l)y making a more elaborate model. 

 The thickeniiiLj of the dielectric layer of a condenser, when its ])lates are 

 Fopavated, corrcspoiuls exactly to the thiekcninj^ and streiifjrtiieidni^ of an 

 clastic membrane ; and rise of potential in the one case is ac-curateiy ropre- 

 Mhtuble by increase of ju'cssure in the other; but such considerations be- 

 imiH' to general electrostatics, and have no special bearing on our present 

 subject. 



J 'J. This is perhaps the most eonvenieut place to introduce the notes or 

 condensed statements which I drew up and disti-ibuted at the meeting 

 htfore the discussion. They were intended to be critically exact (allowing 

 ot Course foi' mistid<esand possible slips) so as to bear aiudysis, and hence 

 it is probably worth while to reproduce them hero with notes and com- 

 ments. 



I. — OiJTnopox >t.\ti:mi:nts i;i:Lii:vi:n r.y the wiutek to hi; tiu'i: in 



llli: I'OKM IIEKi: SKT MOWN, 



A.—Vulht. 



i. Two metals in contact ordinarily acquire o])posite cliarges;' for 

 ir.star.ce. cleui zinc receives a positive charge by contact with copper, of 

 >uch a niiigtutude as would be otherwise ^ivoduced under the same cir- 

 cumstances by an J-l.M.F. of about -S volt. 



ii. Tliis apparent contact E.^[.l"\ or ' Volta force ' is independent of 

 all other mdalUc contacts wheresoever arranged ; heuco the metals can be 

 arranged in a numeiical scries such that the ' contact force ' of any two 

 is eciual to the difference of the numbers attached to them, whether the 

 contact be direct or through intermediate metals. But whether this series 

 <'l;angcs Avhen the atmosphere, or medium surrounding the metal, changes 

 I- ar. open question; on the one side are xperiments of De la Rive, 

 Brown, 8chultze-Berge ; on the other side, of Pfaff, Pellat, Thomson, Von 

 Zahn.- It certainly changes when the free metallic surfaces are in the 

 slightest degree oxidised or otherwise dirty. And in general this ' Volta 

 iorce ' is very dependent on all non-metallic contacts. 



iii. In a closed chain of r.ny substances whatever, the resultant E.M.F. 

 iS the algebraic sum of the Volta forces measured electro.-^tatically in air 



' Olisevve that it L-; not .siid tliat two mi'tals in contact acf|iurc (lilTorcut potoii- 

 tn!>. Such dUVc'ieiua" ()£ iwli'Utial I oclieve to bo unlv anoaivnt. Coinoan' liRs. It 

 aii.l 1.-. ' ^^ 



• I put Vou Zahu on that .side borauso lie iiiiiiself (considers hirasclf tlicrc, and 

 oe'.':insu the great bulk of his experiments lean decidedly that way. 



K K -' 



