Abstract: A system for measuring the volume of a part of a human body such as an arm, for diagnostic purposes, is disclosed measuring instrument comprises an outer tube and an inner tube made of flexible material. Both ends of the inner tube are secured to an inside wall of the outer tube so as to form a watertight chamber between the outer tube and the inner tube. Electrodes are provided upon the inside wall of the outer tube, and the watertight chamber is filled with electrolyte. Electric current is passed from one of a pair of electrodes to another electrode through the electrolyte. Voltage across another pair of electrodes is measured so as to measure changes in impedance, thereby detecting changes in the volume of the body part.