494 report — 1884. 



violent experimental disproof, which no one has ever attempted to give. 

 This fact, that the sum of the 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. But 

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

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

 experimental proof of the summation law, therefore, establishes that air/M 

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

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



8. Leaving electrostatic determinations as without bearing on the point 

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

 the actual E.M.F. at a particular junction without disturbance from other 

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



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

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

 JII is the E.M.F. at that junction acting with the current. This is of great 

 importance, as it is the only method of measuring a local E.M.F., the 

 ordinary method of connecting up by wires to an electrometer being 

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

 contact forces in other parts of the circuit. But the E.M.F. so measured 

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

 Hence the assumption that the potential of a metal is to be measured by 

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

 of Volta's E.M.F. must be sought for, not at the junction of the two 

 metals, but at one or both of the surfaces which separate the metals from 

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



And in another place he says : — 2 



' 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 what 

 is called the E.M.F. of the circuit — a measurable quantity. Of these 

 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 current to pass from 

 copper to zinc, the heat generated in the 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 in over- 

 coming 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 overcoming 

 the E.M.F. from the Zn to the Cu, and the amount of this heat per 

 nnit of electricity is a measure of the E.M.F. Now it is found by 

 thermo-electric experiments that this E.M.F. is exceedingly small at 

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

 zinc to copper. 3 Hence the statement, deduced from experiments in 

 which air is the third medium, that the E.M.F. from copper to zinc 

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

 difference between the potential in air near the surface of copper and the 

 potential in air near the surface of zinc, the zinc and copper being in 

 contact. The number 75 is therefore the E.M.F., in volts, of the circuit 



1 Electricity and Magnetism, vol. i. art. 210. Abbreviated above because so easy 

 of reference. 



2 Maxwell : Letter to the Electricitr.it, April 20, 1870. Also El. Electricity, p. 110. 

 s Further on (sect. 23) I point out that this statement is n I ite ta but it 



does not affect the main argument. 



