A transducer for converting thermal energy directly into electrical energy. It is composed of pairs of thermocouples that are connected either in series or in parallel. The output voltage of N pairs of series-connected thermocouples is N times the voltage developed by a single pair, while the current developed by N pairs of parallel connected thermocouples is N times the current developed by a single pair. Thermopiles are used in thermoelectric radiation instruments when the output of a single pair of thermocouples is not large enough. See moll thermopile, pyrheliometer.