Science Puzzle
How Does a Flask Keep Tea Hot?
Pour tea into a mug and it's lukewarm within the hour. Pour it into a good flask and it's still hot at the end of a long day out. The same flask keeps iced water cold, too.
Cut a flask in half and you'd find two walls, one inside the other, with a narrow gap between them. What's in the gap that makes it work so well?
The Answer
Almost nothing. The gap is a vacuum: nearly all the air has been pumped out before the walls are sealed.
Heat escapes from a hot drink in three ways. It passes from particle to particle through anything touching the drink (conduction). It's carried away by moving air or liquid (convection). And it streams out as invisible infrared light (radiation).
A vacuum stops the first two dead: with no particles in the gap, there's nothing to pass the heat along and nothing to flow. The walls facing the gap are silvered or polished, so most of the infrared bounces back towards the tea instead of crossing over. What little heat escapes leaks mostly through the stopper and the neck, where the two walls meet.
The Scottish scientist James Dewar invented the vacuum flask in 1892, to keep very cold liquefied gases from boiling away in his laboratory. Two German glassblowers turned it into a product for everyday drinks in 1904 and called it Thermos, from the Greek word for heat. It works in both directions, which is why the same flask keeps ice cold: heat can't get in any more easily than it gets out.
The principle: Heat transfer. Heat moves by conduction, convection and radiation; a vacuum blocks the first two and shiny surfaces reflect most of the third.