Abstract: A control circuit for a magnetic probe. A power supply connects the available power to the direct voltages required to control electronic circuitry, and a voltage required for the magnetic probe. A clock establishes a time base for the entire device. A current alternator changes the direction of current flow through the magnet to change the polarity, and a pulse rate control sets the frequency with which the polarity is changed. The current alternator is variable to allow one polarity to remain for a longer time than the other, and to vary the timing for each. The frequency of the pulsing is counted, and the particular frequency is displayed in readable form.