a pair of proximally spaced parallel rotors , the proximal area between the rotors defining a center position containing a center line extending through both rotary axes of the rotors , a plurality of circumferentially spaced magnets of alternating polarity around each rotor and a shielded electromagnet having a pole tip in the center position intercepting the center line , and means for alternating the polarity of the electromagnet as each pair of opposed magnets of like polarity on opposite rotors move through the center position , in order to magnetically draw the magnets approaching the center position and repel the magnets leaving the center position .