The thermocouple

If two dissimilar metals are joined together and the junctions between them maintained at different temperatures an e.m.f. will be generated across the junctions. This e.m.f. is proportional to the temperature difference as long as this is not too large. A measurement of this e.m.f., normally with a potentiometer (see below), will therefore give us a measure of the temperature difference .


Joining an intermediate metal into the circuit will not affect this e.m.f., providing that the points where it is joined are at the same temperature.

These thermometers have the advantage of a very low thermal capacity, they are small and they can be made direct-reading. They can therefore be used to measure varying temperatures and they will detect changes in temperature of about 0.001 ^oC.

The ranges of various pairs of wires and the e.m.f. generated for a temperature difference of 100 ^oC are given below.

Copper/constantan	0oC to +/- 300oC	4.00 mV
Platinum/platinum + rhodium	0oC to +/- 1600oC	0.65
Tron/constantan	0oC to +/- 1090oC	5.30