In practice, a peltier element is a flat pice of metal with two wires sticking out. If you apply a voltage across the wires, the plate gets hot on one side and cold on the other. The cool thing (no pun intended) is that there are no moving parts in it!
If you put the peltier element between a hot and a cold surface, it will instead output a voltage across the wires.
http://www.heatsink-guide.com/peltier.htm http://www.eureca.de/english/peltierelement/info.html An excerpt taken from:
http://www.fys.kuleuven.ac.be/pradem/fysici/Peltier.html
Jean Charles Athanase Peltier (1785 - 1845)
French physicist born in the Somme department of France, Peltier was a watchmaker who gave up his profession at the age of 30 to devote himself to experimental physics. In 1821 T. J. Seebeck had shown that if heat is applied to the junction of a loop of two different conductors a current will be generated. In 1834 Peltier demonstrated the converse effect (the Peltier effect). He found that when a current is passed through a circuit of two different conductors a thermal effect will be found at the junctions. There is a rise or fall in temperature at the junction depending on the direction of current flow.
