The German physicist Seebeck discovered in 1821 the thermoelectric effect which forms the basic of modern thermocouple technology. He discovered that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures. The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions of the Thermoelement circuit The voltages at each junction cancel each other out and no current flows in the circuit if both junctions are held at the same temperature. But with different temperatures at each junction, different voltages are produced and a current flows in the circuit. A thermocouple can therefore only measure temperature differences between the two junctions, a fact that dictates how a practical thermocouple can be utilised.

A more practical way is to compensate the reference junction electronically. The voltage created at the junction between copper - and thermocouple wires (which represents the ambient temperature) is simply added up to the voltage difference between hot and cold junction. e.g. If the measuring junction is at 100 deg. C and the terminal temp.is at 20 deg. C the measured thermal voltage corresponds to 80 deg. C in temperature difference. A positive correction of a voltage corresponding to 20 deg. C referred to 0 deg. C gives the correct result of 100 deg. C at the measuring point. Note that thermocouples are always formed when two different metals are connected together. The term thermocouple usually refers to a complete system for producing thermal voltages and generally implies an actual assembly (i.e. sheathed device with extension leads or terminal block). The two conductors and associated measuring junction constitute a thermoelement and the individual conductors are identified as the positive and negative leg. Developments in theoretical aspects of thermoelectricity under the influence of solid state physics has resulted in a rather different explanation of thermocouple activity.

This is that the thermoelectric voltage is generated in the thermocouple wires only in the temperature gradient existing between hot and cold junctions and not in the junctions themselves. Whilst this is a fundamental conceptual difference to established theory, the way in which thermocouples are currently used is generally successful in practical terms. However, this explanation of thermocouple behaviour must be born in mind when calibrating the sensor or indeed when using it for relatively high precision thermometry. Like mentioned above, different metals create different voltages. A table in the appendix gives the reader an idea of the voltages created by a 'K' thermocouple at different temperatures. It can be easily seen that the voltages change only a few micro volts per deg. C, in consequence thermocouples are usually used at elevated temperatures of several hundreds of deg. C. For lower temperatures the use of other applications like for example a resistance thermometer is more advisable. Different thermocouples cover different ranges of temperature. The thermocouples supplied by Northern Instruments are stated bellow :

• Type 'B' 300 to 1820 deg. CELSIUS for Pt 30% Rh / Pt 6% Rh. ,Type 'B' or 570 to 310 deg. FAHRENHEIT for Pt 30% Rh / Pt 6% Rh. ,
• Type 'K' 0 to 1372 deg. CELSIUS for NiCr / NiAl. Type 'K' or 0 to 2500 deg. FAHRENHEIT for NiCr / NiAl.
• Type 'N' 0 to 1300 deg. CELSIUS for NiCrSi / NiSi. , Type 'N' or 0 to 2400 deg. FAHRENHEIT for NiCrSi / NiSi. ,
• Type 'R' 0 to 1768 deg. CELSIUS for Pt l3% Rh / Pt. , Type 'R' or 0 to 3214 deg. FAHRENHEIT for Pt l3% Rh / Pt. ,
• Type 'S' 0 to 1768 deg. CELSIUS for Pt l0% Rh / Pt. , Type 'S' or 0 to 3214 deg. FAHRENHEIT for Pt l0% Rh / Pt. , Type 'S'