34 
PHYSICS: E. H. HALL 
Graphically interpreted, equation (8) gives for the line AD, of figure 1 and 
figure 4, P D - P A = J- X E'ADD'd'e', where £' is a point halfway between 
£ and A, while £>' is halfway between G and Z>. 
For the potential difference of each of the. lines A B, B C, and D C, of 
figure 1, we have a corresponding representation; but these areas all combined, 
the sum of the areas for A D and D C being subtracted from the sum of the 
areas for A B and B C, will give, as before, ~ X A B C D, as the net e.m.f. 
Ge 
of the circuit. 
T D 
C T 
FIG. 3 
FIG. 4 
If the diagram A B C D represented the operation of an ordinary fluid, like 
air or steam, working in a cylinder under a piston, the path ABC would rep- 
resent that part of the cycle in which the fluid, expanding, does work on the 
piston at the expense of heat energy, while the path C B A would represent 
that part of the cycle in which the returning piston does work in compressing 
the fluid. In the thermo-electric case, under the conditions which we have 
assumed, the path ABC represents that part of the cycle in which the ex- 
panding electric fluid does work in storing up electric potential energy at the 
expense of heat energy, while the path CD A represents that part of the 
cycle in which electric potential energy works to compress the electric fluid. 
