THERMO- COUPLE

When two wires of different metals are joined to make a loop and the junction are kept at different temperatures, a potential difference develops between them. As a result a current flows through wires. This potential difference is measured and used to find temperature difference between the junctions. Thermometer based on this principal are called thermo-electric thermometer or thermo-couples.

The junction when maintained at constant temperature difference. The temperature can be well estimated. This type of thermometer is widely used to measure temperature of the furnace used in a heavy duty heating or high temperature heating.

Simple measurement of thermo-emf using potentiometer (detail can be found in chapter 33 of this book), leads for the calibration and determination of temperature in some cases galvanometer is also used to measure the deflection produced due to thermo emf. The deflection produced at diff temp difference give rise of a calibration curve. Hence with the help of calibration curve unknown temperature of the body can be determined.

Between 630.74ºc and freezing point of gold(1064.43ºc), the international temperature scale is expressed in terms of emf of a thermo couple. Wires used in a thermocouple for the measurement of temperature are platinum and platinum-rhodium(90% pt and 10% rh) alloy. While measuring the thermo EMF using potentiometer special care should be taken. That is the thermo EMF are not set up at the junction of thermo couple wires and copper wire connecting the potentiometer. This effect can be avoided by making three junction as shown in the figure. 15.6. The junction of copper lead and thermocouple leads are maintained at same temperature say either at temp or room temperature and junction of two thermocouple wires is brought in contact with the hot body whose temperature has to be estimated. The EMF E of the whole system system is then equal to the EMF of two platinum, platinum rhodium junctions, one in ice and next at hot body.