Thermocouple

A thermocouple is a junction between two metals, for instance, copper wire is soldered to bismuth wire. The other end of the bismuth is soldered to copper again.

One junction is put in ice-water to serve as a reference temperature. The other is at some higher temperature. A small voltage is developed between the junctions. The size of the voltage is related to the temperature difference. The voltage is measured with a digital meter and a temperature difference is displayed. Thermocouples can be very small, with small heat capacities, and are very useful as thermometers in many applications.

The interested student could make and calibrate their own thermocouple.

If metal bars are used rather than thin wires the resistance in a circuit can be made very low. Even the fraction of a volt generated by a thermocouple may then drive a very large current. This was the principle used by Ohm to drive a large current with a reliable voltage source.

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