A thermistor (short for "thermal resistor") is a type of resistor that is used to measure temperature. While typical resistors are designed to maintain consistent resistance regardless of temperature, a thermistor's resistance varies significantly as the temperature changes. Once a thermistor is calibrated, changes in electrical resistance can be accurately translated into changes in temperature.
Thermistors are commonly used in computers to monitor the ambient temperature of internal components. For example, thermistors may be used to record the temperature near the CPU, RAM slots, and the power supply. These thermistors are usually integrated into the computer's motherboard. The actual temperature of components such as the processor and memory modules is typically measured by a diode that is integrated into the chip.
Computers use the information recorded by thermistors to prevent overheating. For example, if a processor is running near capacity for an extended period of time, the temperature may gradually increase. When this happens, the computer might speed up the internal fans to increase airflow and cool the computer. In extreme circumstances, such as when a laptop is used outside on a hot day, the fans may not be able to keep the computer at a safe temperature. If the thermistors record a dangerously high temperature, the computer may shut down to avoid overheating and damaging the hardware.