  It is proposed to used, as a basic property specifying the difference between
an insulator and a conductor, a static phenomenon, namely the field effect
which absent in the former, but present in the latter. The absence or present
of the field effect is closely associated with the nature of the homogenious
linear response to a static electric field. For an insulator, it is finite; for
a conductor, it depends on the volume V and tends to infinity when V to
infinity. The fluctuation-dissipation theorem makes it possibile to relate the
nature of this response to the mean square fluctuation of the dipole moment of
the system, which is normal in an insulator, but anomalious in a conductor. A
number of examples are considered. A notable property of the BCS
supercondctivity state have remarked, namely: its conductivity property is a
consequence of non-zero electron density fluctuations in the ground state of
the system with ODLRO.
