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☕ How a thermos keeps a drink hot

Heat escapes by touch, by moving air and by infrared light. Follow all three routes through the wall of a vacuum flask — and meet the physicist who invented it for liquid hydrogen and never made a penny from it.

5
lessons
~18 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. Three ways heat gets awayTell conduction, convection and radiation apart, and name which part of a flask answers each.Heat leaves by touch, by moving fluid and by infrared light; a flask attacks all three differently.
  2. What the vacuum actually doesLocate the vacuum correctly and explain what it can and cannot stop.The evacuated space is a moat inside the wall, and it is only one member of a team.
  3. The lid closes the biggest leakConnect the opening, the stopper and evaporation to how long a drink stays hot.The neck is the one route the vacuum cannot close, so the lid does that work instead.
  4. Dewar's cold, Burger's coffeeTrace the flask from cryogenic apparatus to household object, and who profited.Dewar built it in 1892 for liquefied gases and never patented it; Burger's firm made it Thermos.
  5. What a thermos cannot doState the limits of the design and test one of its claims by hand.A flask slows heat rather than stopping it, and a vacuum is an absence rather than a barrier.

Questions this course answers

Where is the vacuum in a vacuum flask?

The drink sits at ordinary pressure; the evacuated space is the jacket in the wall around it.

The two walls of a flask are joined to each other where?

Burger's 1907 patent describes double walls united with each other only at the mouth; anything else between them is a non-conducting spacer.

Why does simply thinning the air in the gap not help much?

Gas conductivity is nearly pressure-independent until the mean free path exceeds the width of the gap.

Which route can a vacuum not block?

Infrared radiation needs no medium, so it crosses an evacuated gap freely.

What are the shiny inner surfaces for?

A polished, low-emissivity surface both emits and absorbs far less thermal radiation than a matt one.

Why is the neck the weak point?

The opening cannot be bridged by vacuum, so it is a genuine conduction path and a route for vapour.

Grounded in trusted sources

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