A thermocouple is a ‘device used extensively for measurement of temperature of a body.’

A thermocouple comprises of at least two metals joined together to form two junctions.

One in connected to the body whose temperature is to be measured; this is called the hot or measuring junction.

The other junction is connected to a body of known temperature of the body with reference to the known temperature of another body.

Working Principle • The working principle of thermocouple is based on three effects, discovered by Seeback, Peltier and Thomson. They are as follows:

1) Seeback Effect • The Seeback effect states that when two different or unlike metals are joined together at two junctions having a difference of temperature, an electromotive fore (e.m.f.) is generated at the two junctions.

• The amount of e.m.f. generated is different for different combinations of metals.

2) Peltier Effect

• As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, e.m.f. is generated within the circuit due to the different temperatures of the two junctions of the circuit.

3) Thomson Effect

• As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.

• In most cases, the e.m.f. suggested by Thomson effect is very small and it can be neglected by making proper selection of metals.

• The Peltier effect plays a prominent role in the working principle of thermocouple.

• The general circuit for the working of thermocouple is shown in the figure.

• It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p and q, which are maintained at temperatures T1 and T2 respectively.

• Remember that the thermocouple cannot be formed if there are no two junctions.

• Since the two junctions are maintained at different temperatures the Peltier e.m.f. is generated within the circuit and it is a function of the temperatures of two junctions.

• If the temperature of both the junctions is same, equal and opposite e.m.f. will be generated at both junctions and the net current flowing through the junction is zero.

• If the junctions are maintained at different temperature, the e.m.f.’s will not become zero and there will be net current flowing through the circuit.

• The total e.m.f. flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions.

• The total e.m.f. or the current flowing through the circuit can be measured easily by a suitable device.

• For measurement of the temperature of the body, one junction of the thermocouple is connected to the body whose temperature is to be measured.

• This junction is known as the hot junction or the measuring junction.

• The other junction is connected to the body whose temperature is known. This junction is the cold or reference junction.

• The device for measuring the current or e.m.f. is connected within the circuit of the thermocouple.

• It measures the amount of e.m.f. flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures.

• Now, the temperature of the reference junction is already known, while the temperature of measuring junction is unknown.

• The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature.

• Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

• Advantages of Thermocouples

• The instrument does not require any external excitation source.

• Design is simple and rugged.

• Suitable for rapid changing temperature as it is more sensitive.

• Cheap as compared to other instruments.

• Disadvantages of Thermocouples

• At higher temperatures it has non – linear characteristics.

• Less accurate as compared to other instruments.

• One junction is required to be maintained at reference temperature.