The oldest direct conversion heat engine is the thermocouple. Take two
different materials (typically, two dissimilar metal wires), join them,
and heat the junction. A voltage, or electromotive force, can be
measured across the unheated terminals. T. J. Seebeck first noticed this
effect in 1821 in his laboratory in Berlin, but, because of a mistaken
interpretation of what was involved, he did not seek any practical
application for it. Only recently has any real progress been made in
using his discovery for power production. To use the analogy of A. F.
Joffe, the Russian pioneer in this field, thermoelectricity lay
undisturbed for over a hundred years like Sleeping Beauty. The Prince
that awoke her was the semiconductor.
As long as inefficient metal wires were used, textbook writers were
correct in asserting that thermoelectricity could never be used for
power production. The secret of practical thermoelectricity is therefore
the creation of better thermoelectric materials. (Creation is the right
word since the best materials for the purpose do not exist in nature.)
To perform this alchemy, we first have to understand the Seebeck effect.