Degaussing

The demagnetization (also demagnetization, in connection with picture tubes also engl degaussing -. Literally Entgaußung ) is a process by which a permanent magnet or a permanent magnetic become ferromagnetic material loses its magnetic polarization in whole or in part.

Method

Demagnetize materials usually only by a strong alternating magnetic field, which then decays gradually. This magnetic field has to be so strong that the coercive field of the magnetized material is achieved. By the alternating field is done a reversal of magnetization of the permanent magnetic materials having a decreasing amplitude. So that the hysteresis curve is traversed with a decreasing amplitude of the magnetic field strength and the magnetic flux density, to the permanent magnetic field is zero. The picture to one example, starts up right, then it is always counter-clockwise to lower left, then back up to the right, but not so far etc. If after many rounds the smallest loop at the intersection of the two axes is reached, you can switch off the external magnetic field, the iron is substantially demagnetized.

For the attenuation of the alternating field, there are two methods: either removed slowly a portable solenoid which is operated at 50 Hz or is placed in series with the coil a PTC thermistor, which can be a low current. According to the manufacturer of Entmagnetisiereinrichtungen to keep it in 5 to 10 magnetic reversals.

Demagnetization may be effected by mechanical vibration or the heating of ferromagnetic materials of the Curie temperature.

Application variants

Tools

Demagnetization is important with tools to prevent their attraction for chips or components or in cases where magnetic fields interfere (eg balance of coils). If you have no so-called non-magnetic tools available, you have to demagnetize if necessary. There are operated at mains voltage electromagnets. Also, permanent magnet, stair -shaped components are in use, from which access form considering the tools and creates a decaying alternating field in them. Such devices can often also be used for magnetization, to magnetize specific tools for assembly tasks.

Magnetic Play and storage media

In analog video tape recorders and tape players, there is an erase head is driven with high-frequency electromagnet. With this it is possible to delete the present magnetization as information on the tape locally to have unmagnetized tape for subsequent recording. In digital magnetic storage media, however, overwriting the information is sufficient.

By using so-called demagnetization Degaussern that generate a large-scale alternating field, magnetic storage media, such as floppy disks, hard disks and magnetic tapes, are rapidly cleared in larger quantities to ensure data security at their disposal.

Even devices and storage systems for magnetic tapes, but also for picture tubes and this itself must be free of permanent magnetization. Therefore, speakers have in cathode ray tube TVs a permanent magnet circuit - unlike ordinary electrodynamic loudspeakers - has hardly a magnetic field around itself or is shielded.

Magnetization and demagnetization also plays a role in many retail security labels.

Picture tubes

Color picture tubes in monitors and televisions contain magnetizable parts ( hole or slit mask ), which, when they have become permanently magnetic, which deflect electrons, resulting in color distortion. The devices demagnetization ( degaussing ) is therefore carried out every time the mains voltage. This is accomplished with a coil that is wound around the picture tube. It is when you turn a so-called Posistor - one equipped with a heating element PTC resistor ( PTC) - switched and generated by self-heating of the thermistor directly to the mains voltage, a decaying alternating magnetic field. The heating element is used to heat the PTC somewhat further, so that the current through the degauss coil becomes zero.

Ships

With the military ships, for example, absorbed by the Earth's magnetic field magnetization must be reduced to make the ships for magnetic mines and torpedoes with magneto harder to find.

The Demagnetizing for German naval vessels are in Wilhelmshaven, on the Kiel Canal and Kiel. The People's Navy of the GDR used for a about 2.8 kilometers east of Vilm created artificial island in rügischen Bodden.

In addition, some vessels have their own degaussing system, not only the permanent magnetic properties of the hull, but other magnetic signatures, such as the electromagnetic field generators, with the offset. There was also earlier attempts should be where placed around the hull coils used for remote detonation of mines.

Magnetically shielded rooms for Biomagnetism

For the diagnostic investigation of magnetic fields occurring during the brain, nerves or heart activity with a field strength of a few pT to nT, it is necessary to shield the stronger by many orders of magnitude magnetic interference fields of the urban environment of the measurement location and the ever-present earth's magnetic field of about 40μT. These magnetic Abschirmräume come from highly permeable material (eg Mu - Metal) used. A prominent representative is the BMSR-2, the magnetically bestgeschirmte area in the world, the Physikalisch- Technische Bundesanstalt in Berlin. These spaces shall be demagnetized regularly to keep the residual field within the space as small as possible and thus allow measurements of fields down to the fT area.

Precision Engineering

Precision engineering structures, eg mechanical movements may need to be demagnetized to prevent unwanted adhesion and to avoid disturbing forces.

Causes the magnetization

" The reasons for magnetization of parts are very varied and can be in practice not always easy to determine. However, are usually artificially generated magnetic fields which act in the immediate vicinity of the workpieces. This may be unintentional nature or intentional origin, reinforce eg solenoid transport, linear oscillator, induction, magnetic grippers, magnetic chucks etc.. Mechanical vibration and cold deformation under the effect of these magnetic fields or favor the process of magnetization. "

Literature

• Günter Springer: electrical engineering expertise. 18.Auflage, publishing - Europe - teaching aids, Wuppertal, 1989, ISBN 3-8085-3018-9
• Richard P. Feynman, Robert B. Leighton, Matthew Sands: Lectures on Physics. 3rd edition, Oldenbourg Verlag, Munich, Vienna, 2001, ISBN 3-486-25589-4
• F. Thiel, A. Schnabel, S. Knappe - Gruneberg, D. Stollfuß, and M. Burghoff: demagnetization of magnetically shielded rooms, Rev. Sci. Instrum. 78, 035 106 (2007 ) (13 pages ), doi: 10.1063/1.2713433