Refrigeration

Abstract

Cooling something to 77 K is conceptually simple: immerse the object in liquid nitrogen. No one worries about where liquid nitrogen comes from; it is a by-product of oxygen production, and costs in truckload quantities. However, the field of superconductivity has long been relegated to the research laboratory explicitly because it is so expensive to reach 4 K, the temperature of liquid helium. The purpose of this chapter is to explain how very low temperatures are achieved.