Bimetallic thermometers are contact temperature sensors found in several forms if you know where to look, e.g. inside simple home heating system thermostats or in an industrial plant.
They’re the coil of metal that has some electrical contacts affixed to it inside a thermostat.
They are perhaps more familiar to many people in industry and commerce as miniature pocket dial thermometers that one can use to check the temperature of fat in a deep frier or a vat on a small process line.
Perhaps the best part is that bimatallic, or bimet, thermometers can be sealed and do not require batteries!
The major uses are where a reliable, unpowered temperature is required or a quick check of the temperature of an object is desired, or, in the case of the thermostats, a simple, reliable, low cost sensor that provides the input and sometimes a good part of the control action in a home heating system.
They are simple, yet have limited temperature ranges and accuracies.
(Nowadays electronic thermostats are more commonly seen and since they need a battery to power the clock inside them, they usually have a different type of temperature sensor, one that also runs with a battery.)
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