How does a thermos flask work?

           The thermos flask is a special kind of bottle in which hot liquids like tea and milk remain hot and cold items like ice, cold water etc. remain cold for a long time. It was invented by Sir James Dewar in 1892. That is why it is called Dewar flask also. It consists of a double-walled glass bottle. These walls are silvered from inner side. 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 its mouth.

          It is interesting to know how this bottle helps in keeping the hot things hot and cold ones cold. Normally a hot substance becomes cold by losing its heat to the surroundings having a lower temperature. Similarly a cold substance becomes hot by absorbing heat from the surroundings having higher temperature. The function of the thermos flask is to prevent the flow of heat to and from the bottle. We know that heat can flow from one place to another by three modes: conduction, convection and radiation. The thermos flask prevents the flow of heat by any of these modes. The flask is made up of glass which is a bad conductor of heat. Hence heat does not flow by conduction. Since there is a 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 a thermos flask do not become cold and cold ones do not become hot for a long time.