Abstract

An apparatus is disclosed which is utilized to provide positional information relative to an external magnetic field. The apparatus includes a rotatably mounted slotted disk wherein the slot is continuously varying in width around the circumference of the disk. The continuously variable width of the slot serves as a light attenuating mask. The disk incorporates an elongated magnet means which causes it to rotate in relation to an external magnetic field. The slotted disk may be placed in a fluid filled chamber. A first and second aperture are located on the top and bottom of the chamber, the apertures defining an emission passage through the chamber. The apertures have a rectangular length approximately the maximum width of the slot. The slotted disk is intermediate the apertures. A source detector, source emitter and an area source means is provided. The area source means is caused to emit a continuous uniform light signal which is attenuated by the slotted disk. The area source provides a uniform light intensity across the apertures. This provides the source detector means with a meaningful signal. In the disclosed embodiments, the source detector does not directly see the emanations from the primary source emitter, hence, it is not neccessary that the source detector be sensitive to the primary source emanations.