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PTC thermistors are typically used in switches. This is because when a specified critical temperature is reached within a device, their resistance spikes and stops current flow. NTC thermistors are typically used in alarming devices, because their ambient temperature-resistance is high. For example, within a fire alarm, the NTC thermistor keeps the amount of current going to the alarm low when there is no fire. However, if a fire occurs, the ambient temperature increases, allowing more current to flow into the circuit, eventually triggering the alarm.