Cryogenics

Studies of the vastness of the universe and the invisible smallness of the atom are remote from the things of everyday life. Scientists also go far from familiar things in the study of low temperatures. Hydrogen liquefies at 252.7DEG below zero Centigrade and helium liquefies at -- 268.9DEG. Compared to such temperatures, the inside of an ice-cream freezer is a seething furnace. The utmost cold, absolute zero (zero on the Kelvin scale), comes out at -- 273.13DEG on the arbitrary Centigrade scale (zero for the freezing point of water, 100 for the boiling point). Scientists have not quite chilled matter to absolute zero, and never expect to. Nevertheless, researches in cryogenics (low tempera ture) are important to the study of entropy, which is defined as the degree of randomness or lack of organization in the energy distribution of a closed system. The third law of thermodynamics states that at absolute zero there must be no entropy. The particles have two kinds of motion, random motion due to the external energy of heat, and their own intrinsic motion. Thus, at absolute zero the random heat motion is obliterated and the intrinsic motion would thus be perfect.

Three major centres of world research in cryogenics are: the University of Leyden in Holland, Oxford University in England, the University of California in the U. S. In California the work is directed by handsome, dapper William Francis Giauque, who first devised the method of cooling magnetic salts closer to absolute zero than had previously been done. His method makes use of the principle that magnetization heats matter, demagnetization chills it. After preliminary cooling with liquid helium, the salt is magnetized, the heat thus generated drawn off into a jacket filled with helium vapor; then demagnetization pushes the substance down one notch further into the cold. By repetitions of this cycle, Dr. Giauque (pronounced gee-oke) has reached one-tenth of one degree above absolute zero. This is the U. S. cryogenic record. Since no imaginable thermometer could record such cold, it is calculated by the Curie law which gives magnetic susceptibility as a function of temperature.

The world record low is held not by Dr. Giauque but by the Dutch scientists in Leyden, who have used his system and reached the astounding figure of .0044DEG. One reason for this is that the Leyden researchers work with magnetic fields up to 27,600 gauss (magnetic units), whereas Dr. Giauque must get along with 8,000 gauss until his university finds the money to string bigger power lines into his lab oratory. Another reason is that Giauque does not regard the pursuit of absolute zero as a competitive stunt, but as a means of studying entropy, and for this purpose the region within a degree of the zero is cold enough. Such study is a great chemical timesaver. For all chemical reactions must obey the laws of thermodynamics. In the absolute zero region information is obtained which shows in advance whether or not certain chemical reactions are thermodynamically possible. If they are not possible, there is no use looking for a catalyst (chemical activator) and otherwise wasting time trying to make impossible combinations.

In the Physical Review last week, Dr. Giauque published a report on the viscosity of liquid helium in the absolute zero region. Viscosity is "fluid friction" or degree of stickiness. Even pure water has some viscosity. But about 2DEG above absolute zero, liquid helium has so little stickiness that -- as several cryogenic experimenters have found -- "rather phenomenal surface films" will spread over any surface brought in contact with it. These films will, in fact, climb right out of the liquid and ascend to considerable heights against gravity.

Dr. Giauque measured the capillary flow of liquid helium through a channel only one-ten-thousandth of one centimetre wide. He obtained such a low value for the viscosity at one degree above that he concluded there would be no fluid friction whatever at the utter cold of absolute zero.

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