How does a thermos work?
A steel flask keeps chai hot from breakfast to teatime with nothing at all: a gap emptied of air, two mirror-bright walls and a good stopper. Block heat's three escape routes in 3D, run twelve hours in twenty seconds, and find out why a fizzy drink should never go in one.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A bottle inside a bottle. A vacuum flask is two bottles, one inside the other, joined only in a thin ring at the neck. The air between them has been pumped out, the facing walls are shiny, and a stopper closes the mouth. All-steel flasks weld two steel bottles together; older ones hold a silvered glass refill in a case.
- Three escapes, three blocks. Heat leaves by conduction, convection and radiation. The vacuum stops the first two: with the 5 mm gap full of air, 90 °C chai would lose about 12 W by conduction, and with a vacuum only hundredths of a watt. Radiation crosses a vacuum, so the walls are shiny: dull walls would leak about 15 W, polished steel about 0.7 W.
- The real leaks: neck and stopper. Where the vacuum can't reach, heat still gets out: along the neck, where the walls are joined, and through the stopper. In a good 500 mL steel flask they carry over half of the roughly 2 W it loses at 90 °C.
- Slow cooling, not no cooling. A flask cools along the same curve as any hot drink, about twenty times more slowly than a jug with a lid. Started at 95 °C in a 20 °C room, a good steel flask is near 75 °C after 6 hours and 60 to 65 °C after 12. Half-full, it cools about twice as fast.
- Making and losing the vacuum. Removing half the air does nothing: conduction only falls once the gas is so thin that molecules cross the gap without meeting, below about 1 pascal. Steel flasks are sealed in a vacuum furnace with a getter to soak up stray hydrogen. If the outside of a flask gets warm, the vacuum has gone.
- Use it safely. Hot drinks in a flask stay hot enough to scald for hours. Never seal in a fizzy or fermenting drink: its gas can push on a 4 cm stopper like a weight of tens of kilograms. Food kept warm should stay above 60 °C, as the WHO advises.
| Term | Meaning |
|---|---|
| Vacuum flask | A double-walled bottle with the air pumped out of the gap between the walls. Often called a thermos, a former trademark that became an everyday word. |
| Conduction | Heat passed on by molecules bumping into their neighbours. A vacuum has almost none to pass it on. |
| Radiation | Heat carried by infrared light, which crosses a vacuum. Shiny walls block it. |
| Emissivity | How strongly a surface glows in the infrared, from 0 to 1: about 0.03 for silvered glass, 0.075 for polished steel, 0.9 for a dull surface. |
| Mean free path | How far a gas molecule travels before hitting another. In a good flask's gap it is longer than the gap itself. |
| Newton's law of cooling | Heat flows out at a rate proportional to the temperature difference, so hot things cool fast at first and then slower. |
| Time constant | Heat stored divided by heat lost per degree: about 20 hours for a good 500 mL steel flask, under an hour for a jug. |
| Getter | A reactive metal sealed into a vacuum to soak up gas that seeps out of the walls later. |
| Danger zone | About 5 to 60 °C, where germs in food multiply fastest. The WHO advises keeping cooked food above 60 °C. |
From a tube of mercury in 1643 to a laboratory bottle of liquid air in 1892, and the steel flask in every Indian office bag.
- 1650 · The first air pump (Otto von Guericke, Magdeburg, Germany)
- 1804 · Shiny surfaces glow less (John Leslie, Edinburgh, Scotland)
- 1892 · Dewar's vacuum flask (James Dewar, Royal Institution, London)
- 1898 · Liquid hydrogen, kept in a flask (James Dewar, Royal Institution, London)
- 1903 · A flask for everyday use (Reinhold Burger, Germany)
- 1904 · The name Thermos (Reinhold Burger, Germany)
- 1963 · The appeal court agrees (US Court of Appeals, Second Circuit, New York, USA)
- 1978 · The first stainless steel vacuum bottle (Nippon Sanso, Japan)
The full story, with 23 moments, charts, people and 19 sources: glassbox.how/e/thermosclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
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glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
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|---|---|
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Which chapters you have opened, your best quiz scores, and sound on or off. |
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/thermos.js |
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