Pack hot coffee in a thermos at dawn, and it's still steaming at lunch. Fill the same thermos with iced tea, and it stays cold all afternoon. It's an old joke: how does the thermos know which one you want?
It doesn't need to know. A thermos simply slows heat from moving in either direction. It's really two bottles, one inside the other, joined at the neck. Most of the air between them has been pumped out, leaving a near-vacuum. Since heat has a hard time crossing empty space, hot things stay hot and cold things stay cold.
Three Ways Heat Escapes
Heat moves in three main ways. Conduction is heat passing through touch, like a hot pan warming its handle. Convection is heat carried by moving air or liquid, like warm air rising from a heater. Radiation is heat traveling as invisible rays, like the warmth you feel from a campfire.
A vacuum flask fights all three. The near-vacuum between the walls greatly reduces conduction and convection, since there's very little air left to carry heat. Many flasks also have shiny, silvered surfaces facing the gap, which help cut down heat lost through radiation.
It isn't perfect. Most of the heat that sneaks in or out does so through the neck and opening, where there's no vacuum. That's why a good stopper matters, and why your coffee eventually does cool off. A bonus with cold drinks is that the outside of the flask barely sweats, because the vacuum virtually eliminates condensation.
Born in a Science Lab
The vacuum flask was invented in 1892 by Scottish scientist James Dewar, who studied extremely cold temperatures. While running experiments, he built a brass chamber inside another chamber and pumped out the air between them to keep his samples at a steady temperature.
Dewar was a giant in cold science. In 1898 he became the first to collect liquid hydrogen, and he produced solid hydrogen the next year. Scientists still call these containers Dewar flasks in his honor.
The Patent That Got Away
Here's the twist. Dewar never patented his invention. In 1904, two German glassblowers, Reinhold Burger and Albert Aschenbrenner, realized it could keep everyday drinks hot or cold. They designed a sturdier flask for daily use and gave it a catchy name: Thermos.
Dewar later tried to claim the rights to his invention but lost the court case to the company. Viennese inventor Gustav Robert Paalen improved the bottle further, and the American Thermos Bottle Company began mass-producing it in Norwich, Connecticut, which brought prices down and put a thermos in countless lunch pails and picnic baskets.
The name became so common that a U.S. court ruled in 1963 that lowercase thermos was a generic word, though Thermos with a capital T is still a registered trademark in some countries.
Still Hard at Work
Vacuum flasks still do serious science, too. Labs use them to hold liquid nitrogen, and MRI machines rely on double vacuum flasks to keep their super-cold liquid helium from boiling away too quickly. Because so little heat leaks in, liquids like these boil away very slowly and can stay liquid for a long time without refrigeration.
So the next time you pour a hot cup from your thermos on a chilly morning, you're enjoying a bit of laboratory science. All it took was two bottles and a whole lot of nothing in between.
