It is defined as the rate of change of thermoelectric emf with temperature.
Mathematically, thermoelectric power at temperature T given by
where dE is the thermoelectric emf developed in a thermocouple having a temperature of hot junction increased by dT from C.
Fig. 1 Thermoelectric Power
The equation of thermoelectric emf is written as
Differentiating both sides with respect to T we get
i.e Thermoelectric power,
It is of the form y=mx+c and, therefore, graph between thermoelectric power and temperature is a straight line. This straight line crosses the temperature axis at a neutral temperature point i.e when
Note: At neutral temperature,
It is the amount of heat absorbed or evolved or evolved in joules at any junction of a thermocouple, when a unit current (one ampere) flows for one second. It is denoted by .
Characteristics of Peliter Coefficient
(i) It is different for different pairs of metals of a couple.
(ii) It depends upon the temperature of junction for a given thermocouple.
(iii) It is numerically equal to the contact different of potential in volts at any junction.
It is the amount of heat absorbed or evolved in joules between two points of the same conductor which differ in temperature by C, when one ampere of current flows for one second. It is denoted by .
Characteristics of Thomson Coefficient
(i) It is not constant but varies with the temperature.
(ii) It is numerically equal to the difference of potential C.
(iii) It is also known as the specific heat of electricity.
THERMOCOUPLES AND APPLICATIONS
As stated in Art 1.2, the combination of two dissimilar metals is known as thermocouple.
Fig . 2 Thermocouples and Applications
Thermocouples are usually employed for measurement of temperatures. This is achieved by connecting an indicating instrument to the thermocouple, as shown Fig.2 and measuring thermo-emf. In this circuit thermocouple acts in the from of energy and indicating instrument as a load. The arrangement shown in Fig.2 is called the pyrometer. Such pyrometers are cheap in cost and can be used to measure temperature up to C.
The materials used in practice depend upon the temperature range to be measured; thus, copper-constantan couples are suitable for temperature up to aboutC, iron-constantan couples up to C and a couple made of platinum and platinum iridim alloy is suitable for measurement of temperatures up to about C .
Thermocouples are also used for operation of relays ar electronic control.