The basic principle of thermoelectric thermometry is that a thermocouple develops an emf which is a function of the difference in temperature of its measuring and reference junctions. If the temperature of the reference junction is known, or compensated for by the use of a reference junction as described in Chapter 7, the temperature of the measuring junction can be determined by measuring the emf generated in the circuit. The use of a thermocouple in temperature measurements, therefore, requires the use of an instrument capable of measuring the output voltage of the Thermocouple circuit.