A thermos uses a vacuum to block heat transfer, keeping drinks at their original temperature for hours.

The vacuum flask was invented by Scottish physicist and chemist Sir James Dewar in 1892 while he was conducting research at Oxford University. Dewar needed a way to keep liquid gases at extremely low temperatures for his experiments. He discovered that placing one glass vessel inside another and removing the air between them created a powerful thermal insulator.Thermal energy typically moves in three ways: conduction, convection, and radiation. Conduction and convection both require a medium, such as air or water, to transport energy. By creating a vacuum, Dewar eliminated these two primary pathways, as there are no molecules in a vacuum to vibrate or circulate.To address the third method of heat transfer, radiation, Dewar silvered the surface of the inner glass. This reflective coating acts like a mirror for infrared waves, bouncing heat back into the container rather than letting it escape. Modern manufacturers like Thermos L.L.C., founded in 1904, still use these core principles to maintain beverage temperatures.While a perfect vacuum is impossible to achieve, high-quality flasks can maintain a vacuum pressure of approximately 10 to the power of minus 3 torr. This level of insulation is so effective that it can keep liquids hot for up to 12 hours or cold for 24 hours. The only significant heat loss occurs through the stopper, which is why modern lids are often made of thick, insulating plastics or silicone.