A thermistor is an economical means of precisely sensing heat over a limited range of temperatures. A thermistor is a Highly sensitive to small changes in temperature, fairly accurate, metal oxide whose change in resistance is typically an inverse function of the change in temperature. An excitation current is passed across the sensor and the voltage, which is proportional to the resistance, is measured and converted to units of heat calibration. Since thermistors usually have a large base resistance and a large change in resistance per unit of temperature change, compensation for lead wire length is not generally needed. This sensor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −75 °C to 500 °F. These sensors are mainly used for automotive purposes.