Anyone who's taken a hands-on course in electric circuits knows that resistors warm when conducting current. The amount of heat is proportional to the product of the resistance and the square of the current. This relationship, known as Joule's Law, was discovered in the 1800's by James Joule an English physicist. Other scientists studying “thermoelectricity” at the time include Seebeck, Peltier, and Thomson. Their findings — that certain materials convert electric energy directly into heat and vice versa — are now employed in everything from submarines to spacecraft to automobiles.
In 1821, German physicist Thomas J. Seebeck discovered a measurable voltage along a metal conductor, hot on one end, cool on the other. The voltage is on the order of several millivolts per degree °C of temperature gradient. This effect, named after its discoverer, is also observed across the junctions of a circuit formed by two metals, where one junction is hotter than the other.
Electrons move from the hot to the cold end of a metal bar, producing a voltage. The voltage is proportional to the temperature gradient as well as fundamental material properties.
In 1834, French physicist Jean C. A. Peltier discovered the inverse of the Seebeck effect: A current passing through the junctions formed by two dissimilar metals or semiconductors creates a thermal gradient, causing one junction to heat while the other cools. The “Peltier effect” is employed in solid-state heat pumps, also known as thermoelectric coolers.
In 1854, Scottish scientist William Thomson discovered that a current-carrying conductor subject to a temperature gradient will absorb or give off heat depending on the material and the polarity of the current. Thomson, who later became Lord Kelvin, showed that unlike Joule or resistive heating, which is not reversible, the heating mechanism he discovered changes sign with a change in the direction of current.