Thomson Johnson Seebeck, a German physicist, Discovered in 1821 that a magnetic compass needle held close to a circuit made of two different conductors was deflected when the two junctions in the circuit were held at different temperature. The deflection means that under the condition describe above, a voltage called the thermoelectric emf (or Seebeck emf or Seebeck voltage ) develops, causing a current to flow in the circuit. The magnetic field due to  this deflects the needle. The two junction circuit, often called a thermocouple .The voltage developed is rather small, of the order of a few micro volts per degree Celsius difference in temperature T₁ and T₂ between the two junctions.

Example :- Experiments on many different pairs of junctions shows that the emf V_AB  for two materials A and B is the difference between two terms V_A and V_B , characteristics of the materials A and B respectively, i. e.

A Schematic circuit for the Seebeck effect. Two different conductors, A and B are joined together at two points or junctions. These are at different temperature T1 and T2. The thermo emf can be measured via a voltmeter inserted as shown.

    V_AB =  V_A  -  V_B

Thermoelectric emf ( Often called thermo emf ) is thus a property of each material, though it is easily measured for a junction of two dissimilar materials at different temperatures. The thermoelectric emf of a number of thermocouples (i.e. conductor junctions) is given by a simple form

      V_AB =  a q +  ( bq² / 2 )

Where  q  is the temperature difference between the junctions, and  a, b  are material parameters.