Construction of a thermos flask.
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Answer:
Humans can be pretty contrary at the best of times. When it's cold, we want to warm up; when it's hot, we want to cool down. That's because we're warm-blooded creatures who need to keep our body temperatures more or less constant, at around 37°C (98.6°F), just to survive. Vacuum flasks are a bit like people in this respect: they like to keep things at steady temperatures. If you put hot drinks in them, they keep them hot; if you put cold drinks in them, they keep them cool. They're simple, neat, and effective
Before we can understand why flasks are so fantastic, we need to understand a bit more about how heat travels.
Heat is a kind of energy that moves around our world in three different ways called conduction, convection, and radiation. If you touch something hot, heat flows straight into your body because there's a direct connection between you and the hot object. Heat conduction happens only when things touch.
Radiation is slightly different again from conduction and convection. When objects are hot, they give off light. That's why camp fires glow red, orange, and yellow. This happens because the atoms in hot objects become "excited" and unstable when they gain extra heat energy from the fire. Since they're unstable, the atoms quickly return to their normal state—and give off the energy they had as light. (Read more about how and why this happens in our longer article about light.) Sometimes we can see the light that atoms produce and sometimes not. If the light they produce is just a bit too red for our eyes to see, it's called infrared radiation and, rather than seeing it, we feel it as heat. You can feel the infrared given off by hot objects even if you're not touching them (so there's no conduction) and there's no air or liquid present to carry heat either (so there's no convection). Radiation explains why we can feel heat coming from old-style, incandescent lamps even though they're surrounded by glass with a vacuum inside.
You can read much more about heat energy in our main article on heat.
Why your coffee goes cold
How a cup of coffee cools through the three heat transfer mechanisms of conduction, convection, and radiation
Artwork: Your coffee cools through a mixture of conduction, convection, and radiation.
Suppose you've just made a hot pot of coffee. You'll be well aware that you need to drink it quickly before it goes cold—but why does it go cold? Boiling water has a temperature of 100°C (212°F), while room temperature is more likely to be 15-20°C (60-70°F), depending on the weather and whether you have your heating on. Since the water in your drink is so much hotter than the room, heat flows rapidly from the coffee pot into the surroundings. Some heat will be lost by conduction: because your coffee pot is standing on a table or worktop, heat will flow directly downward and disappear that way. The air directly above and all around the pot will be warmed by it and start moving around, so more heat will be lost by convection. And some heat will also be lost by radiation.
Together, conduction, convection, and radiation will turn piping hot coffee into something cold, miserable and yucky in less than an hour. If you want your coffee to stay hot, you need to stop conduction, convection, and radiation from happening. And you can do that by putting your coffee into a vacuum flask.
How vacuum flasks work
A vacuum flask is a bit like a super-insulated jug. Most versions have an inner chamber and an outer plastic or metal case separated by two layers of glass with a vacuum in between. The glass is usually lined with a reflective metal layer. Unbreakable flasks do away with the glass. Instead, they have two layers of stainless steel with a vacuum and a reflecting layer in between them. There's also a tight, screw-down stopper on the top.
Thermos vacuum flask
Photo: With the stopper removed, you can clearly see the reflective glass inside this (slightly grubby) Thermos flask.
Reinhold Burger's Thermos vacuum flask patent from 1909
Artwork: Thermos flasks were invented by Reinhold Berger and Albert Aschenbrenner, partners in the German glass-blowing company Burger and Aschenbrenner, who formed the Thermos company to market their idea. Here's one of the original US patents that Burger was granted in December 1907. The basic idea has changed little: vacuum flasks still use a double-walled fluid container, with a vacuum between the walls, to stop heat loss. Artwork courtesy of US Patent and Trademark Office.