James Dewar, the man who invented the thermos flask

Painting of Sir James Dewar lecturing at the Royal Institution

The next time you enjoy a drink from a flask of hot tea, spare a thought for its unsung inventor Sir James Dewar, who died 90 years ago.

The Scots chemist and physicist was renowned in the scientific world of the late 19th and early 20th Century, but his name will be unfamiliar to most people today.

It might have been a different story had Dewar managed to patent his greatest invention - the vacuum flask. He was beaten to it by a German firm called "Thermos" and as a result its name is now known across the world.

Dewar had not realised the commercial value of his 1892 design. He created it for use in his important scientific work researching the liquefaction of gases.

How does a Dewar flask work?

  • Made of two flasks, one inside the other. Air is removed from the space between them to create a near-vacuum. The flasks are permanently sealed together at the neck.
  • Heat conducts poorly through the space between the flasks because there are so few particles to transfer the heat.
  • Most of the heat which enters or escapes the flask does so through the neck or lid, which is usually made of insulating plastic.
  • The inside of the flask is also typically coated with reflective material to reduce heat loss through radiation.

As professor of chemistry at the Royal Institution in London, Dewar had developed a machine that could produce large quantities of liquid oxygen. He needed a suitable container to keep the valuable liquid cool for a long period, allowing enough time for study.

Keeping cool

His answer was the vacuum flask. It was designed as a flask within a flask, joined at the neck, with the space in between evacuated of air.

Having one flask inside another slowed heat transfer by conduction. The near-vacuum between the flasks also prevented heat transfer by convection. Dewar then applied a reflective material to the inside which stopped heat transfer by radiation.

Dewar's invention was perfect for keeping the contents cool, as it blocked the heat coming in from the outside world.

"Ironically, today we use a Thermos to keep hot things hot," says Sir John Rowlinson, Dr Lee's Professor of Chemistry Emeritus at Oxford University.

"Dewar was using his flask to keep liquids cool. In his day he could count on one hand the number of people who would have used it for this purpose, so he didn't see its commercial value."

What's in a name?

The name Thermos has been associated with the commercial vacuum flask to this day. It is so common in fact, in the United States in 1963 Thermos lost the right to use the name to describe its product. It became a 'genericised trademark' alongside aspirin, escalator, cellophane and linoleum.

Sir John is author of the biography 'Sir James Dewar: A Ruthless Chemist'. He believes Dewar's failure to patent his flask would not have been too big a blow to him.

He said: "Dewar was not a man to have vain regrets. He was vigorous in defending his rights, and always insisted on the central importance of his invention. But he seems largely unperturbed by its renaming."

Dewar's flask was manufactured for commercial use in 1904 by two German glass blowers. A contest was held to name their new product and 'Thermos' was chosen as winner. It comes from the Greek word 'therme' meaning 'heat'.

Dewar was unsuccessful in a legal challenge against the new firm. So while he was recognised as the flask's inventor, he never profited from its global success. However, the vacuum flask was just a single triumph in a remarkable lifetime of achievements for Dewar.

Explosive invention

Born in the small town of Kincardine, on the Firth of Forth in Fife, by the age of 12 the clever youngster had learned to build and play his own violin. Both of Dewar's parents died before he was 15, but at the prestigious Dollar Academy he still flourished academically in maths and science.

He entered the University of Edinburgh in 1859 to study arts and chemistry, and became assistant to Professor of Chemistry Lyon Playfair. In 1875 he left Edinburgh for Cambridge where he was elected to the Jacksonian Chair of Natural Experimental Philosophy. Just four later he was made Fullerian Professor of Chemistry at the Royal Institution.

During his work in the field of organic chemistry he described several structural formulas for the compound benzene. For more than 25 years he also studied spectroscopy - the absorption and emission of light.

In the late 1880s Dewar collaborated with the English chemist Frederick Abel on the invention of the explosive cordite - a smokeless alternative to gunpowder used by the British Army in World War One.

Dewar's accolades

  • 1877 - Elected Fellow of the Royal Society
  • 1894 - Awarded the Royal Society Rumford Medal
  • 1899 - Awarded the Hodgkins gold medal of the Smithsonian Institution, Washington, D.C
  • 1904 - Awarded the Lavoisier medal of the French Academy of Sciences
  • 1904 - Received his Knighthood
  • 1906 - Awarded the Matteucci medal of the Italian Society of Sciences
  • 1908 - Awarded the Albert medal of The Society of Arts
  • 1909 - Awarded the Royal Society Davy medal
  • 1916 - Awarded the Royal Society Copley Medal
  • The Dewar lunar crater on the moon was named in his honour

His research into gases led to the liquefaction of hydrogen in 1898. And in 1905 he discovered that cooled charcoal was efficient at absorbing gases and helped create high vacuums. This would become valuable in future research into atomic physics.

Sir John says Dewar's most important legacy is his contribution to cryogenics - the study of the production and effects of very low temperatures.

He said: "Cryogenics is a major field of study today. The liquefaction of hydrogen and his invention of the silvered vacuum flask made this possible. In labs throughout the world the common name for the flask is still 'a Dewar'. A Japanese physicist told me the name is even used in his country."

Vital equipment

While the Thermos ruled the commercial world, Dewar's vacuum flask remains vital to scientists today.

Dr Andy Alexander, Senior Lecturer in Chemical Physics at Edinburgh University, said. "I cannot think of a chemistry lab that does not have one - and more probably several of them. Not to mention the vast majority of physics, biology and medical labs that also use them.

"The flasks are used most often for transporting and containing liquid nitrogen. Liquid nitrogen is used to cool and control chemical reactions for synthesis of new materials and compounds. It is used to trap and liquefy or solidify vapours with low boiling and melting points.

"It is used to chill superconducting magnets in laboratory and medical equipment, and it is used to preserve biological samples such as cell tissues. Dewar's flask is an essential component in every laboratory."

Dewar's work earned him several nominations for the Nobel Prize. He never won the coveted award but received numerous other accolades in his lifetime. He never retired and remained Professor of Chemistry at the Royal Institution until his death on March 27 1923.

While his name may mean little to the average picnicker with their Thermos, it will forever be remembered in chemistry labs around the world.

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