PHYSICS: E. H. HALL 299 
The sum of these two areas, multiplied by (1 -v- Ge), represents the virtual 
e.m.f. in question. 
If the line A D were of constant pv, as it might be if we were dealing with 
an isothermal 'alloy bridge,' the two shaded areas would be equal and their 
sum would equal EA'D'G. That is, there would be no tendency to 'ther- 
mal effusion' in such a case, and the virtual e.m.f. would reduce to 
Ge Jc k 
the value it has under hypothesis (A). 
The total effective e.m.f. for a closed circuit made up of two different 
metals and two isothermal alloy bridges is represented by (1 -f- Ge) times the 
sum of two areas like A" B" C" D" and A\b\ C'[ D'[ in figure 2, where the 
path A BCD represents the pv changes of one gm. of free electrons through- 
out the circuit. 
CL o( 
FIG. 1 FIG. 2 
The total effective e.m.f. of the circuit is not, as I have previously believed 
it to be, necessarily the same under hypothesis (B) as under hypothesis (A). 
For the latter case it is represented in figure 2 by (1 -5- Ge) times the area 
A' B' C' D', the boundary of which corresponds to the changes of pressure 
and volume undergone by 1 gram of electrons, in part free electrons and in 
part associated electrons, in going around the circuit as part of a current. 
I had overlooked the interesting fact that, when we have to do with thermal 
effusion, which overrides the tendency to equality of pressure throughout a 
gaseous body, we can no longer calculate the work done by or on a body of gas 
by means of a mere diagram of its pressure-volume changes. Thus, in a ver- 
tical cylinder containing a column of air, kept warmer at the top than at the 
bottom, a porous partition extending partly across the cylinder wou d con- 
stantly transmit air upward through its interstices, to be returned down- 
ward past the edge of the partition, and work could thus be done by means of 
air acting in a pv cycle of no area. 
