How you can use thermistor in fire alarm?

A thermistor would be a part of the circuit made out of either silcon or geranium.

These are semi-conducting material and have limited electrons at room temp so it has a lot of resistance not allowing much electical current to the 'buzzy thing'. but when they are heated ( by smoke) more electrons become availble and its resistance drops a lot which will increase the eletrical energy for the 'buzz' in the fire alarm