A thermocouple is a really simple 'device' used to measure temperature.
It's been around since 1821 (rather it was 'discovered' in 1821) so the technology has been time tested. It's simple, low cost and works well.
But how does it work? It uses what's called the Seebeck effect (named after the guy who discovered it). It's also sometimes called the thermoelectric effect... but that's another story.
The Seebeck effect basically says that when two different types of metal are joined together there will be a small voltage generated and it will vary depending upon the temperature. The voltage generated will vary depending upon the types of metal as well as the temperature they are exposed to. So, calibration is often necessary.
The interesting part is that the voltage change is going to be small and I mean realllll small. Like under 100uV (as in 0.000100 volts) for every degree Celsius that it changes by. That's a small change and the typical volt meter wouldn't be used to directly measure the change.
Usually, the voltage is amplified and then measured.
A thermocouple can withstand a very high temperature as well as a very low temperature too. Of course, this will vary depending upon which metals we use to create the thermocouple. In general, we can find a thermocouple that will measure as hot as over 2000C and as cold as around -200C. Now that's hot... and cold.
Accuracy of the measurement will also vary depending upon the metal and type of thermocouple used. Typical accuracies we can expect to achieve vary anywhere from around 1/2 to 2-1/2 degrees C.
So, we can join two different metals together and create a thermocouple.
Simple enough. But the observant person that you are probably is wondering how to connect them to our amplifier or meter to actually use them. Wouldn't that wire create another junction of metal? Wouldn't that junction be at a different temperature and cause an error?
The answer is to compensate for that fact. We could soak that second junction in ice water and maintain 0 degrees C. But that's not realistic. So, most thermocouple amplifiers are made just for that purpose and have compensation circuits designed in. Life is good...
For reference, we typically call the point where the two metals meet the hot junction and the other joining point we call the cold junction.
