Abstract: An electromagnetic sensor for measuring signals indicative of a biologic condition in a body channel such as a blood vessel, throat, and the like, comprises a loop-shaped flat frame with laterally compressible and expansible sides, first and second electrodes mounted at opposite sides of the frame, and a third electrode mounted at one side of the frame near the first electrode. Lead wires connected to the electrodes are arranged in a trifilar array and secured to the frame. The lead wires are connected to circuitry comprising a 180.degree. signal inverter for inverting signals picked up at one side of the channel or across the channel to suppress like signals picked up across the channel or at one side of the channel respectively, leaving only desired third signals indicative of the biologic condition to be measured which may be velocity of fluid flow, an esophageal muscular condition, diameter of a blood vessel, etc.