A Peltier is actually another name for what's called a thermo-electric device. It acts as a heat exchanger. It pulls heat off of one side(the processor) and transfers it to the other side(heatsink or watercooler). Great temp differences between the 2 sides can be achieved. The ones ppl usually use for home systems yield about 69 to 75 degrees difference. They run on a DC current. Most times 3 to 15 volts. The more voltage applied(up to the top end tolerence), the greater the temp difference between the two sides. It's based on a principal discovered by a man named, you guessed it, Peltier. He found that when an electric current is passed thru certain metals( bismuth, tellurium, selenium and antimony semiconductors), heat is passed along with the current. I wish I could elaborate more, but unfortunately, Im not a physicist.
Hope this helps...
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