<P> Thermopiles heated by kerosene lamps were used to run batteryless radio receivers in isolated areas . There are commercially produced lanterns that use the heat from a candle to run several light - emitting diodes, and thermoelectrically - powered fans to improve air circulation and heat distribution in wood stoves . </P> <P> Chemical production and petroleum refineries will usually employ computers for logging and for limit testing the many temperatures associated with a process, typically numbering in the hundreds . For such cases, a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the second thermocouple of each circuit . The temperature of the block is in turn measured by a thermistor . Simple computations are used to determine the temperature at each measured location . </P> <P> A thermocouple can be used as a vacuum gauge over the range of approximately 0.001 to 1 torr absolute pressure . In this pressure range, the mean free path of the gas is comparable to the dimensions of the vacuum chamber, and the flow regime is neither purely viscous nor purely molecular . In this configuration, the thermocouple junction is attached to the centre of a short heating wire, which is usually energised by a constant current of about 5 mA, and the heat is removed at a rate related to the thermal conductivity of the gas . </P> <P> The temperature detected at the thermocouple junction depends on the thermal conductivity of the surrounding gas, which depends on the pressure of the gas . The potential difference measured by a thermocouple is proportional to the square of pressure over the low - to medium - vacuum range . At higher (viscous flow) and lower (molecular flow) pressures, the thermal conductivity of air or any other gas is essentially independent of pressure . The thermocouple was first used as a vacuum gauge by Voege in 1906 . The mathematical model for the thermocouple as a vacuum gauge is quite complicated, as explained in detail by Van Atta, but can be simplified to: </P>

Which type of material is used in forming thermocouple