Metal Dewar vacuum flask, early 20th century.
© Science Museum / Science & Society Picture Library
Reproduction. The vacuum flask was invented by the Scottish physicist James Dewar (1842-1923). This is a reproduction of one of Dewar's experimental flasks that has been cros-sectioned to reveal the inside. The flask contains a partial vacuum between its walls that greatly reduces conduction. A perfect vacuum contains no matter and therefore does not conduct heat. The flask is closed to maintain the vacuum and prevent convection, the transfer of heat by air circulation.