Patent GB-1472159-A (GB)
Title: Compensation of magnetic fields of ferromagnetic apparatus due to the earth magnetic field
IPC: B63G9/00
Filed: 1975-06-06
Abstract: 1472159 Demagnetization F FORSTER 6 June 1975 [12 Sept 1974] 24442/75 Heading H1P In ferromagnetic apparatus, e.g. I.C. engines or electric motors for use in, e.g. ships or tanks, the magnetic interference fields produced therein by the vertical component of the earth's field are compensated or minimized after demagnetization of the apparatus in zero field to avoid the possibility of triggering magnetically sensitive weaponry, by applying a compensating magnetic moment opposed to that produced in the apparatus by the vertical component of the earth's field which is derived from at least one permanent magnet, and then treating the apparatus by a vertically directed decaying alternating field of a surrounding loop; the compensating effect being adjustable either by altering the geometric displacement between the magnet and the apparatus surface or by varying the field of the magnet by an adjustable shunt thereon. Compensation is monitored by a differential magnetic field motor having probes acting as a magnetic dipole (Fig. 1, not shown). Further compensating magnets may be applied to the apparatus housing. A diagram may be drawn showing the degree of compensation or overcompensation plotted against the distance of the compensating magnets from the surface of the apparatus.  Due to the screening effect of a hollow apparatus housing, demagnetized internal ferromagnetic parts may be installed therein without impelling the compensation, but the latter may be finely adjusted by horizontal movement of the compensating magnet or magnets over the surface of the housing. Alternatively the assembled apparatus may be compensated while subjected to a decaying alternating field and slowly rotating the rotatable components to avoid polarization of the latter by the earth's field. The empty housing may be overcompensated so that it has optimum compensation when its components are assembled within. When the apparatus is to be used at different geographical latitudes, compensation is repeated for different artificially varied values of the vertical components of the earth's field and the distances between the magnets and the surface is determined for each and plotted one against the other. Then compensation can be varied for different latitudes by adjusting the magnets. In treatment in the decaying alternating field, introduction of magnetic moments due to horizontal components of the earth's field is avoided by balancing out this component with a separately energized coil, or by horizontally rotating the apparatus during treatment. A compensating magnet (Fig. 2) comprises axially magnetized annular permanent magnet 20 whose bore is lined by threaded brass bush 21 screwed on to threaded brass bolt 23 carried by the apparatus housing 24; the magnet being locked with a nut 25 and washer 26. The periphery of the magnet is calibrated at 22 and a flat of the bolt 23 is calibrated at 28 so that the distance setting of the magnet can be recorded. Alternatively a bar magnet may be threaded into a non-magnetic tube carried by the apparatus. In Fig. 3 a compensating magnet is fixed in brass tube 31 carried by housing 24, and a ferromagnetic tube 32 is screwed on to the threaded brass tube to provide an adjustable shunt. A flat of the tube 31 and the periphery of the tube 32 are scale calibrated for displacement of the shunt. Alternatively a ferromagnetic bolt may be adjustably screwed into an annular magnet fixed to the apparatus. German Patent 977,886 is referred to.