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What happens with the electric charges described above
when the thermoelectric circuit is removed from the external heat
baths and temperature relaxes by non-stationary heat flow? Are
the heat conduction equation (34) and the relation
(13) still valid?
For an infinitely extended parallel metallic plate of width
the answer to these questions  is contained in
Fig. 13. The initial surface charges turn into space
charges accompanying the heat conduction process. The interior of
the plate no longer is electrically neutral, but (weakly) charged!