How Does a Thermos Flask Work?

Invented by Sir James Dewar in 1892, the vacuum flask is a masterpiece of thermal engineering. It consists of a double-walled container (usually glass or stainless steel) with a vacuum between the walls. The vacuum is the key: since there are virtually no air molecules between the walls, heat cannot transfer by conduction (molecule-to-molecule contact) or convection (bulk movement of gas). The inner wall surfaces are coated with reflective material (silver or aluminium) to minimise radiative heat transfer (infrared radiation bouncing back rather than passing through). The only significant remaining heat pathways are through the non-vacuum parts: the neck/opening (which is kept narrow), the stopper/cap, and the support structure holding the inner vessel. A well-made vacuum flask can keep liquids hot for 12-24 hours. The same principle is used in cryogenic storage (liquid nitrogen Dewar flasks), spacecraft insulation, and laboratory equipment.