A Thermos Flask Works by Having Nothing Inside the Walls
A flask is a narrow-necked bottle. The vacuum flask keeps drinks hot by removing the air between two layers of glass.
A flask is a narrow-necked container, whether a laboratory vessel, a hip flask or an insulated bottle. The vacuum flask, invented by James Dewar in 1892, works by what is not there: the space between its double walls is evacuated, and heat cannot conduct or convect across a vacuum. Silvering the surfaces blocks radiation too, which leaves almost no route for heat to escape. *Thermos* is a trade name that became generic.
- A narrow-necked bottle. *A conical flask.*
- A vacuum flask. *A flask of tea.*
- A hip flask. *A silver hip flask.*
- Dewar flask — the technical name. *A Dewar flask.*
- Thermos — the trade name. *A Thermos.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun | flask | A glass flask. |
| Plural | flasks | Two flasks. |
| Compound | hip flask | A hip flask. |
| Compound | vacuum flask | A vacuum flask. |
| Named after | Dewar | A Dewar flask. |
- Heat the solution in a conical flask.Laboratory.
- He filled a flask with coffee.British: a vacuum flask.
- Dewar invented it in 1892.History.
Three ways heat escapes, and how a flask blocks them
- Conduction — blocked by the vacuum, which has nothing to conduct through
- Convection — also blocked, since there is no air to circulate
- Radiation — blocked by the silvered surfaces, which reflect it back
- The stopper handles the remaining route, the opening
- Dewar never patented it, so others commercialised the design
Medieval Latin *flasco*, a container, possibly from a Germanic source. English has it from the 1300s.
How does a vacuum flask work?
The vacuum between its double walls blocks conduction and convection, and silvered surfaces reflect radiated heat back.
Is Thermos a brand?
Originally yes. It has become a generic word in many countries, though it remains a trademark in some.