A thermistor is an electronic device that responds to temperature by lowering its electrical resistor. The transistor can be used in a circuit to switch on a fire alarm.
When a fire is lit, the thermistor gets hotter and the resistance of the thermistor decreases. The voltage across the thermistor goes down according to the potentiometer equation
The voltage across the thermistor will fall resulting in a corresponding increase in the voltage across the variable resistor and the base – emitter voltage
across the transistor. When the base emitter voltage rises above a certain level – about 0.6 V - the transistor will act as an amplifier, switching on. When the base emitter voltage is above about 1.5V the transistor is fully on. The above circuit can be used as a fire or heat alarm. Swapping thermistor and variable resistor, and changing the resistance of the variable resistor to a suitable value can make the circuit operate as a frost alarm, detecting when the temperature is too low.