It is our common practice to measure our body temperature with the help of a thermometer when we feel feverish. In ordinary thermometers, the lowest temperature is marked as 0°C. Today scientists can produce temperatures well below 0°C. Kelvin William Thomson a British Physicist first pioneered the absolute scale of temperature. The scale which is used to measure low temperatures therefore called ‘Kelvin’ scale or absolute scale of temperature.
Theoretically the lowest possible temperature which a gas can attain is known as absolute zero. In centigrade scale it is equal to -273.15°C. This is based on the theory that the volume of a gas reduces in correspondence to the fall in temperature. So according to this theory, the gaseous volume would disappear and would loose all its kinetic energy if its temperature was lowered to -273.15°C or absolute zero. The gas molecules would be completely at rest, and it would not possess any heat. In practice, however, all gases change to liquids and then to solids before their temperature reach absolute zero.
Scientists have never been able to reach absolute zero in their laboratory experiments. The lowest recorded temperature so far was achieved by magnetizing copper nuclei at a low temperature. When the electromagnet was switched off, the copper nuclei became demagnetized; the temperature fall was upto a million part of a degree above absolute zero.
Materials react strangely when cooled to a temperature near absolute zero. At this temperature, oxygen gas freezes to a bluish white solid, and a rubber ball becomes so brittle that it shatters instead of bouncing. Mercury, normally a liquid, becomes and shines like hard silver. Hydrogen becomes a liquid and begins creeping into the sides of its container. Natural gas is shipped around the world in special containers after being cooled and liquefied at a low temperature.
The Kelvin scale is used for scientific measurements, e.g. the liquefaction temperature on the Kelvin scale for Hydrogen is 20K and for liquid Helium it is 4.2K.