Recently I've found an article about thermocouple, a simple device that can be used to measure temperature. It uses Seebeck effect where a current in conductor is created if the conductor is heated. The amount of current depends on temperature and type of the conductor.
The thermocouple consist of two different conductors, they have the same temperature, but they create different amount of voltage because they are different materials. A thermocouple measure difference between those voltages.
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.
Without a source of a heat, the multimeter shows 0mV, when the thermocouple is heated, it shows 0,7mV. This is demonstrated on below images.
A small candle was used as a heater, that's why there's a black coating (soot) on the thermocouple.
Since I was only interested in building a working prove of concept, I didn't perform any calibration.
For me it's amazing that such a simple thing can produce quite a lot of energy - it can be directly measured with a multimeter, without any amplification.