chemical elements and their compounds - Sciencemadness Dot Org

Liquid Helium 5
is then pumped off until the vessel has fallen to 1° or 2° K., and the
magnetic field removed. In this way Simon, using a field of 14,000 Gauss,
has reached 0-03° K., while de Haas, with a much stronger magnet, claims
to have reached 0-005° Abs. To realize the result we must remember that
it is the ratio of the temperatures that matters, and not their difference.
The change from 0-005° K. to 4-22° K., the boiling-point of helium, is an
increase in the ratio 844 to 1; another step upwards of the same size would
bring us to 4-22 X 844, or 3,562° K., nearly the boiling-point of carbon.
The success of this process is helped by the facts that the specific heat
of the metallic container is almost negligible at these low temperatures
(even at 12° K. 1 c.c. of helium under 100 atmospheres has as large a
capacity for heat as 1 kg. of copper), and that owing to the low pressure
of the gas (3 X 10~ 16 mm. at 0-2° K.) the vacuum jacket is an almost perfect
insulator.
These temperatures are measured down to about 1° K. by a helium gas
thermometer under low pressure, and below this by means of the magnetic
susceptibility (to weak fields) of the paramagnetic material, which
is approximately proportional to the reciprocal of the absolute temperature.
Liquid and Solid Helium*
In the liquid and solid states helium has properties unlike those of any
other substance.
Liquid helium occurs in two forms, He I and He II, with a sharp transition
point (the A-point) at 2-186° K. under 3-83 cm. mercury 27 ; this falls as
the pressure rises, and the triple point for solid—He I—He II is 1-774° K.
under 28-91 atmospheres. 28 He I (above this temperature) is a normal
liquid; He II, below it, is unlike any other known substance. He II expands
on cooling; it has 10 times the specific heat of He I, but this rapidly
falls; its conductivity for heat is enormous, being found (by the usual
methods of measurement) to be about 3 million times that of He I, and
nbout 200 times that of copper at the ordinary temperature. Again, while
the viscosity of He I is normal, and that of He II as measured by a rotating
disk, 29 though it drops rapidly with falling temperature, retains a finite
Value, the viscosity of helium II measured in a capillary of fine bore or
by flow through a narrow slit appears to be zero 30 or at least less than
10 1l poises.
Htill more remarkably, neither the heat conduction nor the viscosity
©bay what are otherwise universal rules: the heat transport is not
* I am greatly indebted to Dr. K. Mendelssohn for his help in dealing with this
•ttbjoot.
M G. Schmidt and W. H. Keesom, Physica, 1937, 4, 971.
w J. J. van Laar, Proc. K. Akad. Amst. 1936, 39, 612, 822.
•• W. H. Keesom and G. E. Maowood, Physica, 1938, 5, 737.
•• P. L. Kapitssa, Nature, 1988, 141, 74: J. F, Allen and A. D. Misener, Proc. Boy.
fee. 1089, 172, 467.