What is a thermostat?
A thermostat is an instrument which keeps the temperature of a system at a constant level.
It is designed to sense the temperature of a particular system and then perform actions to make the temperature remain at a constant level. The word ‘thermostat’ comes from two ancient Greek words: thermo (which means heat) and statos (which means standing or static). Thus thermostat maintains the temperature at a constant level. 
A thermometer is an instrument to measure the temperature but a thermostat is something that “keeps heat the same.” For example: when our home is too cold, the thermostat in an air conditioner or heating machine switches on the heating to raise the temperature but once the temperature reaches the level that we want (set previously) the thermostat switches the heating off so that we do not feel uncomfortably warm. In this way, we do not have to switch on or switch off manually the heating, and a comfortable balance is maintained. This also saves electricity as needless consumption of power by the heater is prevented.
Principle
Mechanical thermostats use the idea of the concept of expansion to switch an electric circuit on and off. Thermal expansion is the concept where most substances (exception is water) expand when they get heated and contract when it is cooled down. The two most commonly used types of the thermostat are bimetallic strips and gas-filled bellows.
Bimetallic strip thermostat mechanism
Here, two strips of different metals are joined together to form a bimetallic strip.
Examples of metal combinations are
- Brass and Steel
- Aluminum vs Molybdenum.
Usually, the strip works as a wire piece and carries current (in the heating circuit) when the heating is on. When it gets too hot, one of the metal expands more than the other metal and the strip bends breaking the circuit. This stops the current flow and the heating cuts off, allowing the room to cool.
Now as the room cools, the strip cools and bends back to the original shape. So the circuit is now unbroken and current can flow, and heating switches on. We can specify the temperature at which it will switch on or off by manipulating the dial. The heating is not continually getting switched on and off as the bimetallic strip takes some time to expand and contract.
To make the bimetallic strip we can use brass and steel. The linear coefficient of expansion is 19 x 10-6 /°C for brass and 11 x 10-6 /°C for steel. One can also use aluminum and molybdenum. Aluminum has a linear coefficient of 35 x 10-6/°C and molybdenum has a coefficient of 10 x 10-6/°C. We can use any two metals with a significant difference in their thermal linear coefficients. The one with higher coefficient will expand more and the strip will bend away from that side.
Other examples include copper and iron as described in the picture above.
Disadvantages of the bimetallic strip: The biggest drawback of bimetallic strips is that they take a long time to heat up or cool down, so they do not respond instantly to temperature changes. In many cases, this is not desired.
Gas-filled bellows thermostat mechanism
An alternative to bimetallic strip thermostat is the gas-filled bellows thermostat. It senses temperature changes more quickly. It uses a pair of metal discs with a gas-filled bellows in between. The discs have a large surface area so they react quickly to heat and they have ridges in them to make them springy so they can react quickly. When the system warms up, the gas in the bellows expands and forces the discs apart. The inner disc pushes against a micro-switch in the middle of the thermostat turning the electric circuit off. This cuts off the heating allowing the room to cool. As the room cools, the gas in the bellows now contracts and the metal discs are forced back together. The inner disc moves away from the micro-switch, switching on the electric circuit and turning the heating on again.
The application of corrugated disk and bellows is also done using other substances to fill the bellows. They are often filled with volatile liquids or other gases. The substance used within is determined by the temperature range the thermostat will be used in. Diluted alcohol is often used to fill the bellows in such thermostats.
Wax thermostat mechanism
These thermostats use the change in the state of a substance from liquid to gas. Wax thermostats use a little plug of wax inside a sealed chamber. As the temperature rises, the wax melts and changes from solid-state to a liquid state. Thus the volume of the wax increases greatly. Due to this expansion, it pops out a rod out of the chamber that switches the heating off. This will allow the system to cool down and reduce the temperature. In the extreme conditions, wax thermostats are usually more reliable They are longer lasting inside a vehicle engine. These thermostats find application in home radiator valves, car engines and showers. In showers, they can automatically customize the temperature of the water by switching off the heating when it gets too hot.
Temperature valves that are fixed to central heating radiators for warming water use wax thermostats. When the radiators heat up to the preset temperature, the wax melts thus expanding and reduces the flow of water through the radiator until the temperature falls back down again.
Thus we can see that thermostats are a very useful appliance and help us to save both energy and money.