  We present a simple on-chip electronic thermometer with the potential to
operate down to 1 mK. It is based on transport through a single normal-metal -
superconductor tunnel junction with rapidly widening leads. The current through
the junction is determined by the temperature of the normal electrode that is
efficiently thermalized to the phonon bath, and it is virtually insensitive to
the temperature of the superconductor, even when the latter is relatively far
from equilibrium. We demonstrate here the operation of the device down to 7 mK
and present a systematic thermal analysis.
