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Electromagnetic blood flowmeters were first constructed and used by Kolin in 1936 and, independently, by Wetterer in 1937. These early instruments consisted essentially of a large dc electromagnet, a pair of electrodes, and a string galvanometer; they made possible the recording of flow in exposed, large arteries of anesthetized animals under stringent experimental conditions. Modern electromagnetic flowmeters, using tiny, surgically implantable magnet-electrode assemblies and complex-electronic circuits, permit the recording of instantaneous and mean flows in large and small arteries in unanesthetized animals engaged in ordinary activities, as well as in anesthetized human patients whose vessels are exposed during surgical operations. Major steps in the development of the modern flowmeter include: 1) the early use of a sinusoidal ac-excited magnet to obviate the use of direct-coupled amplifiers and cumbersome, nonpolarizable electrodes; 2) the development of demodulation techniques for eliminating from the ac signal those induced voltages unrelated to flow: 3) the introduction of the squarewave and triangular-wave excited magnet to facilitate such signal-from-noise separation; 4) the miniaturization of magnetelectrode assemblies suitable for surgical implantation; and 5) improvements in circuitry resulting in greater electrical stability and freedom from noise.