so heat flows as shown, generating a current $I$, which provides power to a load (battery charger, motor, etc.) that converts the electrical power into some other form of work. The same thermocouple circuit can act as a refrigerator; if one replaces the load with a power supply that generates the current $I$. This induces the heat flow out of Reservoir $L$, which thereby refrigerates Reservoir $L$, so $T_L < T_R$. Note that in both cases the circuit works because the two thermoelectrics are electrically in series but thermally in parallel. In (b), $N$ indicates the number of transverse modes in the narrowest part of the quantum system. ](figure1.pdf){width="\columnwidth"}

The simplest heat-engine is a thermocouple circuit, as shown in Fig. \[Fig:thermocouple\]. It consists of a pair of thermoelectrics with opposite thermoelectric responses (filled and open circles) and a load, connected in a ring. Between each such circuit element is a big reservoir of electrons, the reservoir on the left ($L$) is hotter than the others, $T_L > T_R$, so heat flows from left to right. One thermoelectric’s response causes an electric current to flow in the opposite direction to the heat flow (filled circle), while the other’s causes an electric 