Abstract: A D.C. potential controller is provided for controlling the magnitude and polarity of a D.C. potential applied to a D.C. drive motor. The controller includes a supply device for producing a potential having a predetermined polarity and magnitude and including a first fixed terminal referenced to said predetermined polarity and potential and a second fixed terminal referenced to a reference potential. A regulating element is displaceable between first and second polarity selecting positions, and is further coordinately displaceable from either first or second polarity selecting positions through a third position.