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A thermostat is a device used to regulate the temperature of a place or an object. The word thermostat is originated from two Greek words: ‘thermo’ means heat, and ‘static’ means staying the same.
Thermostats are used in any device or system that gets heated or cooled down to a preset temperature. It turns an appliance or a circuit on or off when a particular temperature is reached.
Devices that use a thermostat include a building heaters, central heaters in a room, air conditioners, water heaters, and kitchen equipment, including ovens and refrigerators. Sometimes, a thermostat functions both as the sensor and the controller of a thermal system.
Thermos flask (Vacuum flask):
The thermos flask or vacuum flask is an insulating storage vessel that maintains its content hotter or cooler than the environment (surroundings) over a period of time. It is primarily meant to increase the storage period of a liquid by maintaining a uniform temperature and avoiding the possibility of getting a bad taste. 
A thermos flask has double walls, which are evacuated, and it is silver coated on the inside of the vacuum bottle. The vacuum between the two walls restricts heat from being conveyed from the inside to the outside by conduction and convection. There is almost no heat transfer from the inner wall to the outer wall with very little air between the walls or vice versa. Conduction can only happen at the points where the two walls meet, at the top of the bottle, and through insulated support at the bottom. The silvered walls reflect radiated heat back to the liquid in the bottle, the same way a space blanket does. The above diagram shows a cross-section through a thermos