An anemometer is for run of wind measurements, or operation with an electronic counter to determine average wind speed. A magnet turns with the rotor spindle and the resulting varying field causes a mercury wetted reed switch to make and break contact once per revolution of the rotor. The contacts are bounce free, simplifying connection to electronic circuits, and no power is required apart from that necessary to detect contact closure, thus facilitating use at remote sites. The rotor is tested by comparison with an N.P.L. calibrated rotor, and a calibration figure is provided.