  We distinguish traditional implementations of autonomous Maxwell demons from
related linear devices that were recently proposed, not relying on the notions
of measurements and feedback control. In both cases a current seems to flow
against its spontaneous direction (imposed, e.g., by a thermal or electric
gradient) without external energy intake. However, in the latter case, this
current inversion may only be apparent. Even if the currents exchanged between
a system and its reservoirs are inverted (by creating additional independent
currents between system and demon), this is not enough to conclude that the
original current through the system has been inverted. We show that this
distinction can be revealed locally by measuring the fluctuations of the
system-reservoir currents.
