A thermos is a household tool used to hold hot or warm water.
The thermos was invented by physicist and chemist Sir James Dewar (1842-1923), a Scottish chemist and physicist, in 1892, thanks to improvements made to Newton’s calorimeter.
Pour a cup of boiling water and leave it in the air, and it won’t be long before that cup of water cools down. However, if you pour boiling water into a thermos, it can maintain the temperature of the boiling water for quite a long time.
The thermos retains heat due to the unique structure of its inner flask.
The hot thermos retains heat due to the unique structure of its inner flask. The inner flask is made up of two layers of thin glass, with the air between the two layers removed and a thin layer of mercury coated on one side of the flask. The opening of the flask is much smaller than its “body”. The opening can be sealed with a soft wooden stopper. This design is what allows the thermos to be a container that keeps heat “warm inside, cool outside.”
Once boiling water is poured into the thermos, the structure of the inner flask prevents heat from escaping through conventional means.
- First, the thermal convection is cut off. The heated air inside the thermos will try to escape, while the cold air outside will seek any gaps to enter. However, due to the relatively small neck of the thermos, which is sealed with a soft wooden stopper, the only pathway for thermal convection is blocked.
- Second, thermal conduction is hindered. Although air conducts heat much less efficiently than metal, heat within the inner flask can still be transmitted through the outer glass shell to the air outside. However, since the inner flask has two layers of glass with a vacuum in between, the air—acting as a thermal conductor—becomes extremely sparse, making the conduction pathway obstructed.
- Third, thermal radiation is completely blocked. In winter, under the sunlight, we feel a bit warm. This warmth is due to the thermal radiation from sunlight. Because the inner flask is coated with a thin layer of mercury, the thermal radiation is reflected back by the mercury layer, preventing it from escaping the flask. This ensures that the pathway for thermal radiation is also completely blocked.
The ideal case is that after cutting off the three types of heat transfer, the hot water in the thermos could retain heat indefinitely. However, in reality, the thermal insulation effectiveness of the thermos is not perfect, so the heat retention has a time limit. Exceeding that limit, the thermos will no longer retain heat.
