Electromotive Forces in the Voltae Cell. 257 
which separate the metals from the air or other medium which 
forms the third element in the circuit.” 
And in another place he says* :-— 
“Tn a voltaic circuit the sum of the H.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 cir- 
cuit—a measurable quantity. Of these three E.M.F.’s only 
one can be measured by a legitimate process, that namely 
from copper to zine. 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 cur- 
rent, for no chemical or other change is effected. Part of this 
heat arises from the work done in overcoming ordinary re- 
sistance 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 H.M.F. from the zinc to the copper, and 
the amount of this heat per unit of electricity is a measure of 
the H.M.F. Now it is found by thermoelectric experiments 
that this H.M.F. is exceedingly small at ordinary temperature, 
being less than a microvolt, and that it is from zinc to copperf. 
Hence the statement, deduced from experiments in which 
air is the third medium, that the H.M.F’. from copper to zine 
is °75 volt, cannot be correct. In fact, what is really mea- 
sured 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 H.M.F., in volts, of the circuit copper, zine, air, 
copper, and is the sum of three E.M.F.’s, only one of which 
has yet been measured.” 
With every word of Maxwell I cordially agree. 
9. 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. Pellat substantially observes as follows :— 
“ Does the apparent difference of potential between two 
metals in contact indicate a real difference of potential be- 
tween them? In all rigour, No! but the slight variation of 
its value when different gases or even liquids are used renders 
it extremely probable that there is such a real difference of 
potential, and that it is very nearly what is measured in elec- 
troscopic experiments.” 
As to difficulties connected with energy considerations 
* Maxwell: Letter to the ‘ Electrician,’ April 26, 1879. Also ‘ Elem, 
_ Electricity,’ p. 149. 
+ Further on (sect. 23) I point out that this statement is not quite 
true, but it does not affect the main argument. 
