
280 The Principles of Thermodynamics
one can go to much lower temperatures where the vapor pressure is still reasonable
enough to use pumping effectively.
The lowest temperature reachable is about 0.25 K which is about a fifth of what
is reachable with He4. It should be noted that as fractions of the characteristic tem-
perature T
v
, the lowest temperatures reachable by both He4 and He3 cryogenics are
comparable, about a tenth. The reason for this should be clear; what limits going to
lower and lower temperatures is the steep fall of vapor pressure with temperature,
and as we have seen, that is controlled essentially by the ratio
T
T
v
.
Though He3 is a