Thermostat is a device that detects any changes in the temperature and maintains or regulates the temperature of any closed region.

We all try to maintain an optimum temperature in our homes and also in our vehicles using different appliances available on market. It can be an air conditioner or a heater. A thermostat is also a similar device that detects temperature changes and maintains the desired temperature within the closed area. When the temperature either falls or increases than the desired value, this instrument releases electrical signals. These signals control the flow of fuel into the burner, i.e., either to the cooling or heating unit. The instrument is also used in a few fire-detection systems. There is a sharp difference between a thermometer and a thermostat. A thermometer only measures the temperature, whereas the thermostat measures and maintains the temperature to the desired level. Let’s study the working and uses of a thermostat in brief.


They are based on the principle of thermal expansion, i.e., matter expands on heating and shrinks on cooling. The appliance uses it to switch on and off the electric circuit. Earlier, these appliances involved mercury thermometers with electrodes. After reaching the desired temperature, the contact was closed by the mercury. However, now, different other sensors are used in thermostats, as mentioned below:

1. Bimetallic Strips or Electrical Sensors 

John Harrison, a clockmaker in the 18th century, invented the bimetallic strip. A traditional thermostat will consist of bimetallic strips formed by two different metals bolted together. This metallic strip converts the change in temperature into mechanical displacement. Since the strip contains two different metals, they both will expand at different temperatures when heated. This difference is responsible for causing the flat strip to bend in one way on heating from the initial temperature. Similarly, it bends in another way on cooling. While heating, the metal having the greater coefficient of thermal expansion remains on the outer side of the curve. Similarly, this metal will remain on the inner side of the curve while cooling. 

Applications of Bimetallic Strips

Bimetallic strip has significant importance in a variety of electrical and mechanical devices as given below. 

  • Mechanical clocks
  • Direct indicating dial thermometers
  • Heat engine
  • Electrical devices such as circuit breakers

2. Gas-filled Bellows

One of the major drawbacks of bimetallic strips is that they take a large amount of time for heating and cooling. This causes delays in reacting to temperature alterations. Thus, an alternative to bimetallic strips is a pair of metal-discs with gas-filled bellows placed in between those discs. TThe gas-filled bellows have a greater surface area compared to bimetallic strips that enables them to respond to any temperature change. Moreover, the ridges present in the discs make them more flexible. 

On heating the room, the rise in temperature causes the gas between the discs to warm up and pushes the discs apart. This causes the inner disc to push against a small switch present in the middle of this thermostat. The push causes the electric switch to turn off. Now, when the temperature drops, the metal discs come close together. Thus, the inner disc moves away from the switch, and the electric circuit gets on again.

3. Wax-Thermostatic Element

Sergius Vernet invented the wax-thermostatic element that is of great significance in automotive thermostats. Using the thermal expansion of waxes, this element transforms heat energy into mechanical energy. These are one of the most popular and common types that can be easily spotted in your house or cars. 

In wax thermostats, the wax starts to melt on increasing the temperature. Thus, on expansion, the wax will push the rod out of the chamber to either switch on or off something. These are one of the most used thermostats and are highly reliable. For instance, the temperature valves of the heating radiators use these types of thermostats. 

Digital Thermostat 

The digital thermostat, also known as a thermistor, is a simple device to measure the temperature. It is a form of resistor with a microcontroller that measures the resistance and also transforms it into a real temperature reading. Compared to regular thermostats, digital thermostats have programmable settings that allow you to program the device as per your requirement. You can set it to warm up the room for a few hours and then automatically turn it off. 


A thermostat is an appliance that checks the temperature change in the surroundings and maintains the desired temperature accordingly. The instrument can send electrical signals to maintain the optimum temperature. They are based on the principle of thermal expansion, which means the matter will expand on heating and will shrink on cooling. Thus, this phenomenon will help in switching on and off the circuit. Digital thermostat is a form of resistor with a microcontroller that measures the resistance and also transforms it into a real temperature reading. It also has programming settings.

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