ABSTRACT
This chapter discusses three systems: a magnetic system; a rubber band; and a reversible electrolytic cell. One of the exciting frontiers of physics is the temperature region close to the absolute zero. Although the technique of magnetic cooling was first used in the 1930s, it is the standard technique used to achieve the very lowest temperatures. The weak magnetic coupling between the neighbouring electron spins splits the states of the ground level so that they are not actually degenerate. The magnetic ion, containing a number of unpaired electron spins, has been written in italics in each case. It might be asked why one uses such apparently obscure compounds. The answer is that the non-magnetic part of the compound acts as a dilutant, keeping the magnetic atoms well separated and so reducing the interaction between them. This ensures that the fall off in the entropy due to this interaction does not occur until very low temperatures, as required.
