  Thermal channels -- the free processes allowed in the resource theory of
quantum thermodynamics -- are generalised to thermal instruments, which we
interpret as implementing thermodynamically free quantum measurements; a
Maxwellian demon using such measurements never violates the second law of
thermodynamics. Further properties of thermal instruments are investigated and,
in particular, it is shown that they only measure observables commuting with
the Hamiltonian, and they thermalise the measured system when performing a
complete measurement, the latter of which indicates a thermodynamically induced
information-disturbance trade-off. The demarcation of measurements that are not
thermodynamically free paves the way for a resource-theoretic quantification of
their thermodynamic cost.
