How Does A Thermos Know To Keep Hot Things Hot And Cold Things Cold?

The joke is that the teller doesn’t know how a thermos works, but it’s a fairly common question and there’s no shame in not knowing how all of your crockery works. Besides, the answer is pretty interesting.

Unless you are the type who thinks the talking crockery section of Beauty and the Beast was a documentary, you know that thermoses do not know when they are supposed to keep things hot or cold. So the question becomes: how do thermoses keep hot things hot and cold things cold? That’s a little easier to answer, with some good old-fashioned thermodynamics. 

Thermoses are pretty commonplace today, and largely used for keeping, for example, baked beans warm (yes, we’re a British publication, get over it). But like the microwave was originally invented for heating up dead hamsters, thermoses emerged from non-food-related research.

In the late 19th century, Scottish chemist and physicist James Dewar developed a machine capable of yielding large quantities of liquid oxygen. The problem was that liquid oxygen boils at around –183°C (–297°F), and there was no way to keep it cool long enough to study it. 

He was fighting the law of thermodynamics (as you should), specifically, the second law, which dictates that heat always flows from hotter to colder areas. Yes, we know the second law says a lot more than that, but that 19th-century understanding is sufficient for talking about the device that keeps your beans warm.

Heat is lost to the environment in three ways; conduction, convection, and radiation. Conduction is the heat transfer that happens through direct contact, and energy transfer through collisions between molecules and atoms. Convection takes place in fluids, as hotter fluid rises to the top of a system and cooler fluid sinks, creating convection currents. Radiation, meanwhile, takes place through electromagnetic waves emitted as atoms move around or vibrate.

When you place hot coffee in a cup, heat is lost to conduction as the atoms within your cup collide with the sides of it, and the air above, transferring some of their energy to it. Convection helps speed things up, delivering hotter fluid to the top for its turn to be cooled until the temperature of your cup of atoms matches the surrounding environment.

Dewar came up with a way to significantly slow down these processes, though in his case it was to keep a liquid cool. In 1892, he came up with a simple, though ingenious, solution. He simply placed one flask within another, separated by a near-vacuum. The vacuum layer prevents conduction and convection, while inside the flask, he applied a reflective material to slow radiation.

The system slows the liquids or beans within from being heated by the outside environment, or cooled by it if you prefer your beans hot. Dewar did not end up patenting the system, and improved, smaller flasks soon became known by the name “Thermos” after the company obtained a patent for it.