7A RESEARCH COMMITTEES. 
name for it, were the phrase less cumbrous. The term “free 
energy” has, however, become well established, and will therefore 
be used in the rest of this report. 
We will represent the free energy by F, so that 
F =U —T@q; 
we see at once that F depends only on the actual state of the 
system, since U and ¢ do so depend. 
4. Consequences of Equation (B), Electromotive Force and 
Thermal Chemistry. 
The first point to be noted here is that we have no direct 
relation between E on the one hand and the differential coefficients 
of U on the other—i.e., we cannot deduce E from thermochemical 
data alone.* We require, in addition, to know the temperature 
of the cell and the entropy changes brought about in it by the 
passage of the current ; in fact, we can only calculate the e. m. f. 
of arrangements for which we can determine beforehand the rate 
of variation of F. 
On the other hand, the converse problem—viz., the deduction, 
from the electrical behaviour of the cell, of the rate of loss of 
intrinsic energy during the passage of a current—is often 
important, and can always be solved, since, as we proceed to show, 
this rate of loss may be deduced from a knowledge of the e. m. f. 
and its temperature variation. 
Since F=U —T 4, 
SOE cy OW nat Soh. 
“sn on 8 Tow! 
but, by equations (3), 
6U do 
ens UTM sate 
Ou Ty 
6k 
Vine 
6 
or T & ey 
* In other words, Lord Kelvin’s equation (A) is only true provided 
do == (0) 
oq i 
a fact which at once explains and disposes of the long-standing difficulty as to the difference 
between the so-called ‘‘ chemical” and ‘‘ voltaic ” heats. 
