Thermister, thermocouple or liquid crystal.
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A thermistor is a type of resistor with resistance varying according to its temperatureThe word is a portmanteau of thermal and resistorSamuel Ruben invented the thermistor in 1930, and was awarded U.SPatent No2,021,491Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elementsAssuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then: ΔR kΔT where ΔR change in resistance ΔT change in temperature k first-order temperature coefficient of resistance Thermistors can be classified into two types depending on the sign of kIf k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistorIf k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistorResistors that are not thermistors are designed to have a k as close to zero as possible, so that their resistance remains nearly constant over a wide temperature rangeThermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metalsThe temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range
