The concept of “low temperature” has been different at different times. In the days when air was first liquefied by Cailletet and Pictet temperatures of 90 to 50° K were considered as extremely low. At the present time, however, since liquiefiers are commercially available, which give the possibility for relatively inexperienced people to liquefy helium in reasonable quantities, few cryogenics physicists would consider the temperature of liquid hydrogen (20 to 14° K) as being “low”. The liquid helium range extends from 4.2° K to roughly 1° K (see Sect. 2) and the aim of this article is the discussion of the region of still lower temperatures which, at the present time, is considered as “very low”.
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