A hot drink doesn't stay hot for long in an ordinary mug, because heat keeps escaping. It escapes three ways. It seeps straight through the cup (conduction), it gets carried off by warm air rising away (convection), and it beams out as an invisible heat glow (radiation). A thermos is clever because it blocks every one of those routes. It has two walls with an empty gap between them — and an empty gap has nothing inside to carry heat, so the seeping and the air-carrying both stop. Then the inside is given a shiny mirror finish, which bounces the heat glow back into the drink. Turn each barrier on and off in the simulator and watch how fast the drink cools.
Most people think a thermos somehow keeps the drink warm by adding heat or trapping it like a blanket. In fact it adds no heat at all. It just shuts the only three doors heat can use: a vacuum gap stops conduction and convection, and a silvered lining reflects radiation back in.
What's actually happening
It feels like a thermos must contain some special warming trick, but it does nothing of the sort — it adds no heat at all. All it does is make it very hard for heat to leave, and to do that you first have to know the only three doors heat can use. The first is conduction: heat creeping molecule to molecule straight through the solid wall of a cup, which is why a metal mug burns your hand. The second is convection: the warm air and liquid at the surface rising and being replaced by cooler stuff, carrying heat off in currents. The third is radiation: every warm thing glows with invisible infrared light, silently beaming its energy out into the room.
A vacuum flask shuts all three doors with two ideas. Its wall is really two walls with a gap between them, and that gap is pumped down to a near-vacuum. Conduction and convection both need matter to carry the heat across, and a vacuum is as close to no-matter as you can get, so almost nothing crosses the gap — this single trick is the heart of the flask. But radiation doesn't care about a vacuum; infrared sails across empty space exactly as sunlight crosses the void to reach Earth. So the second idea is a mirror: the facing surfaces are silvered, and a shiny surface is a poor emitter and a good reflector, bouncing the drink's own glow back into the drink instead of letting it radiate away.
With all three routes choked off, the numbers get dramatic. An ordinary mug of tea is lukewarm within fifteen or twenty minutes; a decent vacuum flask can keep that same tea hot for six hours or more, losing only a few degrees an hour. The very same construction works in reverse to keep cold things cold, which is why a flask holds ice all day — heat is simply blocked from getting in. James Dewar built the first one in 1892 to store liquefied gases in his lab, and the design has barely changed since. It's also exactly how a vacuum-panel cool box and the gap in double-glazed windows fight heat loss: take away the air, and you take away two of the three doors.
A thermos adds no heat; it just blocks all three escape routes at once, which is why it holds cold things cold just as well as hot things hot.
- 1Pour the same hot water into a plain ceramic mug and a vacuum flask, and pop a thermometer (or just a clean finger, carefully) in each.
- 2Check both every ten minutes for half an hour — the open mug drops fast while the flask barely moves, because the flask has shut conduction, convection and radiation all at once.
- 3For a fairer test, wrap the mug in a towel: it slows the cooling a little by trapping air, but still loses to the flask, whose vacuum gap beats any blanket.
Common questions
No, it adds no heat at all. It simply makes it very hard for heat to leave, slowing the loss so much that a good flask can hold near-boiling tea for six hours or more.
Yes. Blocking the three heat routes works in both directions, so heat is stopped from getting in just as it is stopped from getting out, which is why a flask holds ice all day.
The sealed gap between two window panes cuts conduction and convection the same way a flask does. It is often filled with still argon, which carries heat even more reluctantly than air.