About thermoelectric-effect instruments or thermocouples
Thermoelectric-effect instruments rely on the physical principle that, when any two different metals are connected together, an e.m.f., which is a function of the temperature, is generated at the junction between the metals. The general form of this relationship is:
This is clearly non-linear, which is inconvenient for measurement applications. Fortunately, for certain pairs of materials, the terms involving squared and higher powers of T (a2T2, a3T3, etc.) are approximately zero and the e.m.f.– temperature relationship is approximately linear according to:
Wires of such pairs of materials are connected together at one end, and in this form are known as thermocouples. Thermocouples are a very important class of device as they provide the most commonly used method of measuring temperatures in industry.
Certain of these show reasonable linearity over short temperature ranges and their characteristic can therefore be approximated by a series of straight-line relationships for use in intelligent instruments containing thermocouples. In general, however, the temperature indicated by a given e. m. f. output measurement has to be calculated from tables. The set of these tables corresponding to all the standard types of thermocouple available are known as thermocouple tables.