When two different conductors are placed in contact, electrons flow from one to the other if the energy levels of the electrons are different in the two materials. The higher energy electrons cross the junction until the energy levels are the same on both sides. The principle is the same as charging a battery.
If the conducting materials are connected in a loop, equal numbers of electrons will cross both junctions from the higher to the lower side, creating equal but opposed voltages. Since there is no net voltage, no current flows around the loop.
A thermoelectric element is such a loop made from two conductors whose energy levels change at different rates when the temperature changes. If the junctions are not at the same temperature, there are unequal differences in energy levels across the junctions. Thus, unequal numbers of electrons have to cross the junctions and unequal voltages are established. Since there is a net voltage around the loop, a current will flow. The SNAP-27 has 442 thermoelectric elements. The hot junctions are heated inside the RTG. The eight vanes of the radiator cool the cold junctions to 275°C (530°F).
Alan B. Shepard cast his shadow over the Apollo 14 SNAP-27 as he photographed the Apollo Lunar Surface Experiments. In the background is the central relay station, which transmits data from the sensing instruments to the Earth.
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