Johan (and others),
A nuclear physicist's explanation of magnetism seems to be requested, so here goes.Magnetism is a property of bulk material, and is due to the orientation of electrons in materials. Each electron has a magnetic moment and can be viewed as a small magnet. Materials contain huge numbers of electrons, but most materials are not magnetic because the electrons pair up with others that have opposite magnetic moments and cancel the effect (so most materials are “diamagnetic” – no net magnetic effect). Placing a diamagnetic material (eg, wood, plastic, leather) in a magnetic field has no observable effect on the material. Ferromagnetic materials are materials in which the magnetic moments of the electrons behave cooperatively over short regions of the material (electrons are said to be aligned magnetically). These materials are strongly attracted to a magnetic field because each group of aligned electrons (a “magnetic domain”) aligns itself with the magnetic field. Last time I worried about this, it was still a research topic why this cooperative behavior happens, but experimentally it is observed. Ferromagnetism is normally associated with iron (“ferro”), but there are several other magnetic materials (mostly metals), including some of the rare earth materials, and the metals nickel and cobalt. Ferromagnetic materials are used to make the magnets used in our vintage magnetos. Some ferromagnet materials retain magnetism better than others (permanent, compared with temporary, magnets), and this is due to the combined magnetic properties of the materials forming the magnet. Soft iron is a good example of a temporary magnet – it acts like a magnet in the presence of a permanent magnet, but looses that property when removed. Development of permanent magnets was a very important part of magneto development. Steel alloys were found which had good “magnetic retentivity”, including iron-chromium and cobalt steels, and these were used until about 1938, when the development of an aluminum-nickel-cobalt alloy of steel (known as AlNiCo steel) was perfected and began to be used in magnetos. Magnetic retentivity is key to a good magnet. Retentivity is affected by heat (affects action of the electrons and causes many to loose their magnetic coupling), and shock (like dropping a magnet). AlNiCo (and its various versions, AlNiCo-V is common and later magnetos use –VIII, even better) have excellent retentivity. Magnets prior to AlNiCo will profit from an occasional charging, depending on their history, while AlNiCo magnets do regain strength somewhere around 10-15% (my measurements) after charging, so most of us who repair magnetos go ahead and charge the AlNiCo rotor. It takes a strong magnet charger to generate the magnetic field strength, though, so most homemade chargers will not be adequate. This post is way too long now - if you have further questions, I'll try to answer in another post.
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