THERMOCOUPLES

Thermocouples are perhaps the most simple and most widely used devices for measurement of temperature.It essentially consists of two dissimilar metal wires A and B,insulated from each other but welded or brazed together at their ends forming two junctions,as shown in fig a.
when one end of each wire is connected to a measuring instrument,as shown in fig b,it becomes an accurate and sensitive temperature measurement device.
The operation of a thermocouple is based on see back effect,i.e.,if two wire of different metals are joined together ateach end and form a complete  electric circuit then current flows in the circuit when the two junctions are kept at different temperatures.This current is caused y an emf,called the thermo electric emf, set up in the circuit and is a function of temperature difference of the two junctions(the hot junction,whose temperature is to be measured and the cold or reference junction which is maintained at a constant temperature).The thermoelectric emf set up so is the same for any particular temperature,and is not afffected by the size of the conductors,the area in contact,or the method of joining them.