It was invented by Sir James Dewar in 1892. it consists of a double-walled glass bottle. The walls are silvered. The space between the walls is evacuated with the help of a vacuum pump and sealed. This bottle is protected by a metal case. A cork is fitted on the mouth.
It is interesting to know how this bottle helps in keeping the hot liquids hot and cold liquids cold. The function of thermos flask is to prevent the flow of heat to and from the bottle.
We know that heat can flow one place to another by three processes. They are conduction, convection, and radiation. The thermos flask prevents the flow of heat by any of these processes.
The flask is made up of glass which is a poor conductor of heat. Hence heat does not floe by conduction. Since there is vacuum between the walls of the bottle, flow of heat by convection is also ruled out. Due to the silvering of the walls heat is not lost by radiation. As a result, hot things kept in thermos flask don't become cold and cold ones do not become hot for a long time.
