494 



hkport — 1884. 



! 'j S 



u 



!':■); 



violent experimental disproof, whicli no one 1ms ever attempted to give. 

 This fact, that the sum of tho Volta effects equals the sum of the true 

 forces, in a closed circuit of any conducting materials, has nevertheless 

 caused persons to suppose that air/metal forces are negligibly small. IJiit 

 it is clear that they may have any value they like without affecting tho truth 

 of the law. They could only affect it if air /M were not equal to — M/air. Tin; 

 experimental proof of the summation law, therefore, establishes thatair/!M 

 is equal to — M/air, as well as the important fact that tho contact force at 

 each junction is independent of all other junctions of what kind soever. 



8. Leaving electrostatic determinations as without bearing on tho point 

 at issue, let us ask, Is there no direct and straightforward way of mcasurinf; 

 the actual E.M.F. at a particular junction without distui'banco from other 

 junctions ? Tho answer is most clearly given by Clerk Maxwell, thus : — ' 



' Sir W. Thomson has shown that if II is the coefficient of Peltier effoci 

 or tho heat absorbed at tho junction by unit current in unit time, then 

 Jn is the lO.M.F. at that junction acting with the curi'ent. This is of greiit 

 importance, as it is tho only method of measuring a local E.]\[.F., th 

 ordinary method of connecting np by wires to an electrometer beiuL' 

 useless. This Peltier measurement is quite independent of the effect of 

 contact forces in other parts of tho circuit. But the Iv^M.F. so measureil 

 does not account for Volta's force, which is far greater and often opposite. 

 Henco tho assumption that the potential of a metal is to be measured l)v 

 that of the air in contact with it must be erroneous, and the greater piiv: 

 of Volta'.s E.^I.F. must be sought for, not at the junction of the U\" 

 metals, but at one or both of the surfaces whic^h separate the metals from 

 the air or other medium which forms the third element in the circuit.' 



And in another place he says : — - 



' In a voltaic circuit the sum of the E.M.F. 's from zinc to electrolyte, 

 from electrolyte to copper, and from copper to zinc is not zero, but is wlmt 

 is called the E.!M.F. of the circuit — a measurable quantity. Of tlie.-e 

 three E.M.F.'s only one can be measured by a legitimate process, that. 

 namely, from copper to zinc. If we cause an electric cui'rent to pass from 

 copper to zinc, tho heat generated in tho conductor per unit of electricity 

 is a measure of the work done by the current, for no chemical or other 

 change is effected. Part of this heat arises from the work done i;i ovci- 

 coraing ordinary resistance within the copper and the zinc. This part 

 may be diminished indefinitely by letting the electricity pass very slowly. 

 The remainder of the heat arises from the work done in overcomiiiir 

 the E.M.F. from tho Zn to the Cu, and the amount of this heat ]vr 

 unit of electricity is a measure of the E.M.F. Xow it is found by 

 thermo-electric experiments that this E.3[.F. is exceedingly small nt 

 ordinary temperature, being less than a microvolt, and that it is froiii 

 zinc to copper. •* Hence the statement, deduced -from experiments in 

 •which air is the third medium, that the E.]\I.F. from copper to zine 

 is "75 volt cannot be correct. In fact, what is really measured is the 

 difference between the potential in air near the sui-face of copper and tlio 

 potential in air near the surface of zinc, tho zinc and copper bcins? "' 

 contact. The number "75 is therefore the l'].M.F., in volts, of tho lircui; 



' FJcctricitii and Magiwtism, vol. i. art. 241). Abbreviated above because ^o (':i>,'' 



of reference. 



- Maxwell : Letter to tho Ehuiric'i-nu April 2<;, 187!). Also j:i. L'/rrfnri/>/. p- 1^'' 

 ' Further on (sect. 23) I point out that tins statement U not quite true, but ;i 



docs not atfect the main argument. 



>' !' 



