Harry Barratt of Hackney Primary School, London, England, asks :-

How does a thermos flask keep my Dad's tea hot or his fruit juice cold?

John Campbell, a solid state physicist at the University of Canterbury, responded.

A thermos flask is often called a vacuum flask, or a Dewar flask after Sir James Dewar, the scottish scientist who invented it in 1892 to allow his recently discovered liquid oxygen to last longer. Just one example of how useful, everyday, objects come from scientific research.

The Dewar flask was designed to minimise the heat flow out of, or into, it. Heat is transported by three mechanisms: conduction, convection and radiation. To minimize conduction along the container it is made from a poor conductor, usually glass or, if robustness is required, thin stainless steel. The glass needs to be one which has very little thermal expansion with temperature, for example pyrex as used in oven-safe glassware, to avoid it shattering when a very hot or very cold liquid is placed inside. To minimize heat conduction through the thin container we use two layers, like in a double-glazed window, but separated by a vacuum space to prevent thermal convection. To minimze heat transfer by radiation, the inner walls are coated with a shiny (reflecting) film of silver or aluminium.

I have used the same pyrex/silvered thermos flask for 40 years without failure. Larger metal ones, such as those housing a superconducting magnet as used in hospital magnetic resonance imaging (MRI) devices are so efficient the liquid helium needs to be topped up only two or three times a year.

In my research, which needs the lowest temperature in New Zealand, I have three thermos flasks, one inside the other. The outer pyrex/silvered one contains liquid nitrogen at minus 196 degrees Celsius. It may be better to switch to the temperature scale we use in physics, the Kelvin Temperature Scale for which zero is the lowest temperature possible (0 K or about minus 273 Celsius) and, like the Celsius scale, with a hundred degrees between the temperature at which water freezes (273 K or 0 C) coexist and at which water boils (373 K or 100 C).

The pyrex/silvered thermos flask in the liquid nitrogen (77 K) holds liquid helium (4.2 K), which, when the energetic atoms are pumped away with a vacuum pump, is cooled to 1.5 K. Inside the liquid helium thermos flask is the third thermos flask which is made of metal. It contains a small quantity of helium 3 isotope, which, when pumped by a self-contained vacuum pump, cools down to 0.3 Kelvin to allow an incredibly sensitive heat-detector to work.