How does a thermos work?
A thermos jug or bottle works on a very simple principle--the insulating property of air. A thermos bottle has two walls between which there is an empty space. Both the inside and outside walls are usually made of materials that do not transfer heat well. Thus, the hot or cold contents of the thermos bottle stay hot or cold while the outer layer of the bottle and the air between the two layers slow down heat transfer either in or out of the bottle.
"And then the thermos broke!"
Anyone over the age of 50 can remember how carefully the original thermos bottle had to be carried, and there is scarcely a family without a broken thermos story. Old thermos bottles were Thermos bottles--the technology was patented. The outer Thermos wall was made of metal, and the inner wall was made of glass. While the metal wall provided fairly good accident protection, an accidental drop--or even a too-enthusiastic washing--could break the inner glass wall. In some cases, one could order replacement glass liners from the manufacturer, dismantle the gaskets at the top and put in a new liner, but that meant a genuine delay between picnics.
"And the lemonade was still cold."
The thermoses we buy today are much safer to handle. Plastic, by and large, has replaced both the glass on the inside and the metal on the outside. Sometimes Styrofoam is inserted between the two layers for added insulation. It is still possible for serious campers to locate metal-cased thermoses for frequent camping trips.
Whatever the materials, the technology remains the same: a double-walled vessel with an air space in the middle. Simple but wonderfully effective for keeping hot food hot and cold food cold. What would a lunch box be without a thermos?