ELtrriltOlIOTIVK FOUCKS IN THE VOLTAIC VVA.L. 



49-> 



give. 



tl'UO 



theless 

 1. lint, 

 10 truth 

 iv. Tlu- 

 ,tair/^' 

 force at 

 loever. 

 ho point 

 easuruicr 

 )m other 

 ins : -|- ' 

 ier eit'oft 

 me, then 

 s of j^rcat 

 M.F., thr 

 ter l)eiiv_' 



etfoct (it 

 meas\n'eil 



1 opposite. 

 ;asuretl hy 

 eater pav; 

 )f the tw 

 ictals from 

 Liircuit.' 



'Icctrolytc. 

 ut is whn' 

 Of these 

 locess, that, 

 o pass from 

 \ olcctricity 

 1 or other 

 e vA ovcv- 

 Thls part 

 ery slowly. 

 Ycrcomi"- 

 9 heat poi' 

 la foiuul hy 

 y small fit 

 It it is hm 

 rlments "> 

 er to wn^' 

 red i3 tl^'' 

 er and tk 

 er being ii' 

 the civcui! 



U true, but '.' 



copper, zinc, air, copper, and is the sum of three E.!M.F.'s, only one of 

 which has yet been measured.' 



Witlx (ivery word of ^[axwell I cordially agree. 



!». While on the theoretical aspect of the subject it may be well to see 

 what Pellat, as one of the best experimenters on it, has to say. I'ellat 

 substantially observes as follows : — 



' Does the apparent ditference oi' potential between two metals in 

 contact indicate a real ditl'erence of potential between them ? In all 

 rigour, No ; but the slight variation of its value when different gases or 

 even liquiils are u.^cd render.-* it extremely i)robablo that there is such a 

 real dill'erenco of potential, and that it is vei-y nearly what is measured 

 in electroscopic experiments.' 



As to difhcLilties connected with energy considerations and mere 

 contact, he refers to Jlelmlioltz,' and Clausius,-' \\\\n, ho says, relieve him 

 of all responsibility on tliis head. 



The I'aet that the A'oltaie oi'der of the metals is much the same as their 

 order of oxiilisability mnst have struck ncai'ly everybody, iiud must also 

 have been felt as a dithcuity by the upholders of the ellicacy of mere 

 contact. Pellat considers he disposes of it thus : — ' Since the E.M.F. of u, 

 pile is that represented by chemical action, and since by experiment voltaic 

 contact forces hav(; much the sunie values as the E.M.F. of piles, it follows 

 that there is some vague relation between A/Ji and the heats of com- 

 bination, say of substitution of one metal for another in a salt (as in a 

 Daniell).' 



He sums up his experimental conclusions as follows : — 



(1) ' Two diiferent metals united inetallically are covered, in the state 

 of equilibrium, with electric coats of unequal potential. 



(2) ' This difference of potential only depends on the superficial coat of 

 metal. It changes notably when the surface is mechanically scratched, 

 becoming always more positive. As the scratching etfect disappears with 

 time so dues the extra difference of potential. The state of polish of the 

 surface is immaterial, but traces of foreign substances, forming a coat so 

 thin as to be invisible, are able to modify the value of the observed effect 

 enormously. 



(;!) ' The effect depends somewhat on temperature. 



(4) ' The pressure and nature of the gas surrounding the metals have 

 a very distinct but extremely feeble influence, but, since the effect produced 

 is a lagging one, it is probably due to some secondary cause, and it is 

 probable that the difference of potential is really independent of the gaseous 

 dielectric. 



(•">) 'The difference of potential between the electric coats on two 

 metals united metallically has the same value as the E.]\[.F. of an element 

 of a liquid pile formed by these two metals, provided that the E.M. P. is 



' DU: Erhaltun;/ (hr Knift, p. 17, wlicro Iftliiilioltz develdiis Vi^ltirs diiLiiuiil liypo- 

 'licsis iiliinu an attraction (it iiiattfr fur iloctrit'ily, of an iuuouiU <l('])('iulinu; on the 

 kiiiil (if matter, so that it p'ts pulled one way or another acro.'^s a jiuictiou of two 

 (lisisimilar substances. He points out that the Volta effect is explained if zinc be 

 ^'ranted a stronijcr attraction for electricity than copper has. This view he returns to 

 ill liis Faraday Lecture 1881, where also ho refers to Berzelins' electrical theory of 

 clioniical atliuity. The opinions of Profes.sor Ilclinholtz are too weighty to be merely 

 lefcirccl (() in a footnote, but we may have occasion to consitk'v tiiem later. 



'' Die meohanische Jtihandliiiuj dvr Ijlcctrinfiit, chap, V'i. §^ 2 and ;j, when- 

 Prof. Clausius follows up the above idea by considering the lu'ilc which heat plays in 

 the matter, and thus hypothctically explains the Peltier effect also. 



