A magnetic flowmeter in which a pair of metering electrodes is disposed at diametrically opposed points in a flow tube through which flows the fluid to be measured, a magnetic field being established at right angles to the direction of flow by an excitation winding. The winding is energized by a cyclical voltage derived from an a-c power line, the polarity of the cyclical voltage being reversed at a rate which is a sub-multiple of the line frequency whereby each interval between successive points of reversal contains the same number of full cycles and the polarity of the last half cycle in each interval is the same as the first half cycle in the succeeding interval. The arrangement is such as to cancel error voltage components resulting from line frequency disturbances.