Dewar flask

A Dewar flask is a double-walled vessel with the space between the walls evacuated and the facing surfaces silvered. It defeats all three modes of heat transfer at once. The vacuum removes conduction and convection, because there is no medium between the walls to carry heat across. The silvering reflects thermal radiation back, leaving only the small conduction path through the neck. A liquefied gas can therefore sit in an open-topped Dewar on a laboratory bench for hours, boiling away slowly enough to be worked with.

This turned cryogenics from a stunt into a science. Before the flask, a liquefied permanent gas was a fleeting mist to be photographed and lost; afterwards it was a material that could be stored, poured, measured, and used as a bath in which to run other experiments. Kamerlingh Onnes's sixteen-hour helium liquefaction of 1908 would have been impossible without it — the liquid-hydrogen pre-coolant would have evaporated long before the run finished. Every subsequent cryogenic technique presupposes it.

The design is unchanged in principle in the vacuum flask sold for carrying coffee, and scaled versions hold the liquid helium in MRI magnets and the liquefied natural gas in tankers. Its one weakness is the neck, which must physically connect the walls and therefore conducts; research Dewars minimise it with long, thin, low-conductivity necks, and often nest one Dewar inside another so that the outer bath shields the inner one.

James Dewar built the first vacuum flask at the Royal Institution in 1892 and used it to hold the liquid hydrogen he first produced in 1898. He never patented it. In 1904 the German firm Thermos GmbH commercialised the design; Dewar sued and lost, the court finding that he had published freely and could claim no priority. The flask in a schoolbag today is his invention, made by a company in which he never held a share, and he was reportedly not gracious about the outcome.