Abstract: Apparatus for measuring the rate of flow of a fluid in a conduit includes a receptor assembly arranged on the external surface of the conduit, and a removable external connector assembly including an electromagnet for establishing a magnetic field that extends transversely across the conduit, and a plurality of circumferentially arranged electrodes that are insertable into direct contact at their tips with the fluid in the conduit via sealable gland passages contained in the receptor assembly. A calibration adjustment arrangement is provided for adjusting the calibration factor of the flow signal processing system as a function of the conduit cross-sectional area. In a first embodiment, the glands are self sealable, whereupon the gland passages are automatically closed when the electrodes are removed therefrom.