Abstract: In deriving the impedance of a zone of a body errors arise when currents used stray from the zone due to movements of, and/or changes within the body. In the invention a current used for deriving impedance is passed through a zone under investigation using current electrodes outside the zone. A potential also required for deriving impedance is available at two potential sensing electrodes at ends of the zone. The current is confined to the zone by passing control currents just outside the zone between pairs of control current electrodes to set up virtual current barriers minimum along the boundaries of the zone. Pairs of potential sensing electrodes straddling the boundaries are used to adjust currents passed by the control current electrodes to establish the positions of the boundaries and to stabilize these positions when movement and/or internal functional change occurs.