Thermocouple is a simple device to measure temperature by using Seebeck effect where a current is created in a heated conductor. The amount of current depends on temperature and the material from which the conductor was made. It's a common tool when measurement of relatively high temperatures is needed, also a lot of altimeters can measure temperature by using a thermocouple that usually is part of the multimeter set.
The thermocouple consist of two different conductors placed in the same temperature, because each one is made from a different material, the voltage that they produce differs. A thermocouple measure difference between those voltages.
In this post, I will present a simple version of a thermocouple, that I made by myself. If you like the idea of building your own sensors (even just for fun), you might be also interested in articles about home-made photo-element and DYI pressure sensor
I've tried built this sensor as simple as possible, so I used iron nail and a copper wire as parts of the thermocouple. I used a multimeter to measure voltage created by the thermocouple.