The magnetic flux of a magnetic body is controlled with voltage pulses from an active voltage source. The voltage pulses are applied to a winding circuit which is magnetically coupled to the magnetic body. An electronic switch means is used to generate the voltage pulses from a d-c power supply. A control circuit receives information about the winding current, and utilizes known loop impedances to optimize the voltage pulses for optimal magnetic control. Utilizing voltage pulses rather than a continuously variable voltage source has the advantage of lower cost and improved energy efficiency for many applications. The invention may be utilized for measurement of a-c current and d-c current utilizing ordinary current transformers. The switched-voltage control that is disclosed also may be used to facilitate accurate measurement of a-c current in a self-powered manner, with the power supply for the electronics deriving its power from the current transformer secondary circuit.