Recording thermometer
The coil on the thermometer coils up when its hot and down when cold.This is the best tool to use when you need to check things over time.
Thermocouple
In the thermocouple their are two twisted wires that change the amount of electrical current when at different temperatures.It will know when to turn on and off due to temperatures.
Infrared thermogram
Shows different colours for different heat signatures
Binetalic strips
Two metal strips joined firmly together,as it is heated it expands and coils,when cooled it reverses.