Properties of Liquid Helium
Keywords:Liquid, Helium, Vacuum, Electromagnets
This article is about unusual properties or behaviour of liquid helium and its uses.
The normal boiling point of helium is 4.22 K. Keesom in 1932 cooled it to 0.71 K by making it boil under reduced pressure of 0.0036 mm of mercury, but could not freeze.
Liquid helium has a number of unusual physical properties. These are:
Its density is maximum at 2.2 K, being 0.1462 g/c.c. . With the decrease of temperature, the density decreases and tends to be constant below 1.5 K.
It is now established that liquid helium undergoes a transition at a temperature of 2.186 K. This temperature is known as the λ-point.
The liquid helium is called Helium I above the λ-point and is termed Helium II below it.
Interestingly, the two forms of liquid He are strikingly different in their physical properties. For instance, He II has an extraordinarily high thermal conductivity, unusually low viscosity. The liquid creeps along a surface rapidly so much so that a flask partly lowered into the liquid will gain or loose He II as if the flask was porous. He atoms move practically unhindered through He II — a process analogous to superconductivity where electrons move through metals practically unhindered. Below 1 K, He II appears to lose its abnormal properties. Between 1 to 2 K, it resembles no other liquid, gas or solid.
Thermal physics —A.B. Gupta , H.P. Roy.