How Thermoses and Flasks Work

Written by rae williams
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How Thermoses and Flasks Work
Strong insulation means hot liquids in a Thermos will stay hot even in extremely cold conditions. (Thermos image by Vladislav Gajic from

Thermoses and similar flasks are a common sight on morning commutes, where they keep coffee and tea hot for people on their way to work. Originally invented to hold liquefied gases, Thermos flasks have been keeping hot things hot and cold things cold for more than 100 years.


The Thermos flask, originally named the Dewar flask, was invented by a Scottish scientist named James Dewar in the late 1800s. However, he neglected to patent his invention, which led to it being sold by a German company named Thermos, which is where the name comes from today. All flasks of this type tend to be referred to as "Thermoses" because, while it is a brand name, the company that owns it lost the right to its exclusive use in 1963.

How It Works

Thermos flasks work by using a vacuum to keep liquids at a desired temperature. A Thermos is essentially one small flask suspended inside a bigger one, with an airless space between them. This vacuum is unable to conduct heat, allowing cold liquids to stay cold and hot liquids to stay hot. The sides of the inner flask are usually painted with a reflective coating to prevent heat transfer by radiation, which also keeps the temperature stable for longer.


Some Thermos types use glass as the insulating material for the inner flask, which can shatter if it is dropped. In addition, although Thermos flasks insulate liquids very well, heat transfer does still occur, albeit much more slowly, so if you leave a liquid in a Thermos for long enough, hot liquids will get cold, and cold liquids will get warm.

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