How Heat Produces Electricity?

The first source of external energy which can separate electrons away from their parent atom is HEAT. It is most common way of using heat for this purpose is to apply it to the junction of two dissimilar metals. If an iron wire and a copper wire are twisted together to form a junction, for instance, and if that junction be heated, an electric charge will result.
The amount of this charge will depend on the difference in temperatures between the junction and the opposite ends of the tow wires. The greater this temperature differences the greater the charge and vise versa.

This type of junction called a "THERMO- COUPLE". It will produce electricity for as long as heat is applied to it. The thermo-couple more efficient then the twisted wires are used. They are constructed of two pieces of dissimilar metal riveted or welded together.
It is not at present possible to obtain larger quantities of electric power by heating thermo-couple, through investigation on these lines is being actively pursued. Thermo-couple are, however, used in devises whose purpose it is to measure heat (in which they are made to operate directly a meter marked in degrees of temperature), and in other similar of instrument.