The connection between heat and electricity can be clearly observed in the so-called Seebeck effect, named after its discoverer Thomas Seebeck. To understand this effect, let us first consider a thin wire made of some material, for example, iron. If one end is heated, the electrons there move faster and therefore require more space. On the cooler side, there is a slight excess of electrons, which results in a small potential difference between the two ends.
To measure this potential difference, each end of the wire must be connected to another material, such as copper. In this case, a so-called thermocouple is obtained. The potential difference arises from the temperature difference between these two contact points and the different electron concentrations in both materials.
Analogous to the electrochemical series, one can create a table of positive and negative Seebeck coefficients for the so-generated thermoelectric voltage, where the voltage is proportional to the difference in these coefficients and the temperature difference between the junctions:
\[
U = (S_2 - S_1)\,\Delta T
\]
Thermoelectric voltages are generally small and lie in the range of a few microvolts per kelvin.
The reverse effect can also be demonstrated: if an external voltage source is connected to the two copper wires, one junction is observed to cool down while the other warms up. This is essentially the reverse of the previously described effect and is called the Peltier effect.
Both effects are used in industry, for example to convert waste heat into electrical energy or in portable coolers for keeping drinks cold. However, the efficiency of energy conversion (the efficiency) is generally not very high.
Another application of thermoelectric voltage is found in non-contact infrared thermometers. Here, many thermocouples are connected in series to generate a sufficiently large voltage. The resulting voltage is then approximately proportional to the temperature of the object being measured.