PURPOSE: To prevent occurrence of crosstalk, by providing a partially spherical shell, which is symmetrical to a rotating shaft and whose curveture center coincides with rotating center of a universal coupling, and attaching a magnet tightly to this partially spherical shell.

CONSTITUTION: A rotor 5 is fixedly connected to the partially spherical shell 6 through a rotray shaft 2, and these as a whole are rotated on gimbals 3 being a universal coupling. The partially spherical shell 6 is made of an electric conductive material, and curveture center of the spherical shell coincides with rotating center of the gimbals 3. There are two pairs of electromagnets 7 in the neighborhood of the spherical shell 6. The electromagnets 7 are fixed to a base plate 9 through a fixed frame 8 in such a manner that the spherical shell 6 is located in their iron core gap and that they immediately cross one another and are symmetrical to one another. When the rotor 5 moved in perpendicular direction to the rotary shaft 2, the spherical shell 6 always pases iron core gap of the electromagnets 7.