A system for measuring electrical impedance particularly of biological tissues to provide an indication of the condition of such tissue. The system comprises a source of constant-amplitude current and three pairs of electrodes, of which the first pair constitutes excitational electrodes, the second pair constitutes sensing electrodes, and the third pair constitutes focusing electrodes to focus the current flowing between the two excitation electrodes to the region occupied by the sensing electrodes. The system further includes synchronized switching means for alternately connecting the first group and the second group of electrodes to the excitation source.

Apparatus and a method for determining the internal impedance of a portion of an animal body. Two electrical currents of different frequencies are passed through the body portion and the voltage across the body portion corresponding to each current flow is sensed. In-phase coherent detection is performed to remove the out-of-phase signal components and the resulting in-phase voltage signals are subtracted from each other to yield a measure of only the internal impedance of the body portion. The invention is especially suitable for use in measuring the changes in the amount of fluid in the human lung.

A flexible backing pad bears a spaced pair of recording electrodes at right angles to a spaced pair of stimulating electrodes. The recording electrodes may have associated signal ground electrodes. The array is attachable to mammalian skin over a muscle so that the recording electrodes pick up electrical muscle activity. A resulting signal is fed to processing apparatus which generates a stimulating signal and feeds it to the stimulating electrodes. The electrode array is arranged to minimize direct influence of the stimulating signal on the recording electrodes.

An electrode structure for use with medical electronics instruments such as electromyographs is described. A thin, flexible body of non-conductive material has one or more wells therein. A flexible conductive member which provides an electrode is disposed at the bottom of each well. The spacing between a plurality of electrodes, which can provide bipolar and ground inputs to the medical electronic instrument, is precisely determined by virtue of the disposition of the electrodes in the wells. The electrode structure provides contact with a body surface, usually the skin. To facilitate the contact a conductive jelly is used. This conductive jelly is received in the wells. It makes contact with the electrodes. Ribs are provided in the regions between the electrodes which form a seal at the skin so as to prevent the flow of conductive fluid between electrodes; thus preventing short circuits. The flexibility of the structure provides for comfort and reliable long term attachment and also for maintaining the contact of the electrodes and sealing ribs with the skin as the skin and muscle beneath, flex.

The sensor has four electrodes arranged on a common base, three of which are made as closed circuits, placed one into another, whereas the fourth electrode is placed inside the smallest circuit. The external and the central electrodes form a pair of current-feeding electrodes, whereas the electrodes disposed between them form a pair of measuring electrodes. The second design option of the sensor has three electrodes, two of which are made as closed circuits placed one into another, whereas the third electrode is placed inside the electrode that is smaller. The external and the central electrodes form a pair of current-feeding electrodes, and the electrode arranged between them together with the external or the central electrode form a pair of measuring electrodes. The design of sensors makes it possible to use them in combination with biological signal sensors of non-rheographic modality, for example, pulse wave, temperature. The sensor may be incorporated in wristwatch or bracelet.