The MP-981 is a magneto-type detector using a hall element which is suitable for revolution (r/min) measurement from low speed to high speed. Magneto-type detectors use an internal hall element, a permanent magnet, DC amplifier, and voltage regulator and respond to flux, so that a rectangular wave form is derivable from much lower speed all the way though high speed. In addition, a signal indicator is provided in spite of the detector's compact design, making verification of operation and mounting position possible. Meanwhile, the AP-981 is the water and acid proof type, which conforms to Class ' of JIS C0920.