Electromagnetic device with dust-tight enclosure

摘要

An electromagnetic device which comprises an excitation coil wound to assume a substantially cylindrical shape having an axially extending hollow space; a plunger made of a magnetic material accomodated within the hollow space for movement in an axial direction of the excitation coil between retracted and projected positions; yoke means made of a magnetic material and engageable with one end of the plunger to provide a magnetic path which may be interrupted in response to the movement of the plunger towards the projected position, said yoke means extending between the respective ends of the excitation coil and positioned external of the said excitation coil to provide a closed magnetic circuit including the plunger and the magnetic path; a permanent magnet made of intermetallic compounds of cobalt and one more rare earth elements disposed in the closed magnetic circuit so as to develop a magnetic force of attraction attracting the plunger onto the end of the yoke means opposed to the end of the plunger and holding the plunger in the retracted position, said magnetic force of attraction being substantially cancelled by a magnetic force of the excitation coil when the excitation coil is energized; and means for covering the respective ends of the plunger and the yoke means so as to prevent powdered magnetic material from adhering to the respective ends of the plunger and the yoke means.