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The problem of design of a square-wave electromagnetic flowmeter may be simplified by the use of a mechanical chopper as the gating device. Such an instrument has been assembled using commercial components costing less than $1000. A commercial 400-cycle chopper amplifier, Offner model 190, was modified to perform the sampling. Carrier signal was provided by Tektronix 162 and 161 generators. Proper synchronization with the chopper is obtained by use of the 161 delay control. Magnet assemblies were made of ferrite toroidal cores and platinum electrodes, cast in epoxy resin and powered directly by a Heathkit 70-watt amplifier. Using a carrier frequency of 400 cps, good frequency response, stability, and signal-to-noise levels were attained. Calibration curves are linear for forward and backward flow. Sensitivity was sufficient to record blood flow levels of 1 ml per minute.