Thermistors have mainly physical applications in thermodynamics and has allowed the user to control the desired heat output from the heating source as opposed to wood fires in the early ages.
Thermistors often take advantage of the physical properties of materials such as the thermal conductivity and coefficient of thermal expansion, where a material's expansion or contraction varies linearly with temperature.
At a certain temperature threshold two conducting materials may come into contact and switch the appliance on or off.