Everyone has heard the term "Thermos", but most don't know much about why it works and where it came from. A good "Thermos" keeps your food or beverage hot or cold for an extended period of time.
Over a century ago in 1892 Sir James Dewar, a chemist and physicist, invented the food thermos container. They were called Dewar Flasks. The first vacuum flasks commercially made were by a German company named Thermos in 1904. In 1963 the word "Thermos" was declared a genericied trademark. Today most do not ask for a vacuum flask, but instead ask for a thermos. Simply put, a thermos is a vessel which keeps the contents hot or cold by placing an empty space between the contents and the environment. The vacuum (empty space) is used for the thermal insulation. The contents are not in a vacuum condition. The contents are held in an inner flask.
A typical vacuum flask is made of plastic, glass, or metal with hollow walls. Think of two thin walled mugs, one inside of the other. The air is removed from the narrow space between the inner and outer walls. They are sealed together at the neck. The vacuum thus created minimizes heat transfer by either convection or conduction. Radiative heat loss can be minimized with the use of a reflective coating. Sir James used silver.
Heat is transferred by conduction, convection, and radiation. A large fire is an example of all three. The ground for several feet underneath the fire gets hot. This is conduction. Another example is to heat one end of a metal bar. Soon the other end is also hot. The second heat transfer source is convection. Convection occurs because when a liquid or gas gets hot, it tends to rise above the rest of the thermos mugs. Think of the smoke and flames rising above the fire. The third source of heat transfer is radiation. The fire's heat you feel on your face is infrared radiation. Even though you are away from the fire, you can feel that heat. A thermos is made to minimize the transfer from all three sources.
The inner wall of the thermos is usually thin and has a low thermal capacity. Therefore, little heat is lost or gained when food or liquids are added. Some heat or cold is transferred where the two stainless coffee mugs are joined. The stopper is the area with the most potential for loss or gain. Originally the stopper was cork. While cork is still used, most stoppers are now plastic or rubber. To help retain the desired temperature, it helps to preheat or precool the thermos.
Does you thermos know if the fluid or food inside is hot or cold? Does it care? No. All your thermos is doing is limiting the heat transfer through its walls. This lets the contents maintain temperature for an extended period of time. Amazing invention isn't it?