T.O. 33B-1-13-53.1.6.2 BarMagnet.3.1.6.2.1If the shape of the magnet is straightened, as shown in Figure 3-5, a bar magnet is created. The bar magnet has poles ateither end and magnetic lines of force that flow through the length of it and return around the outside of it. Magneticparticles will be attracted only to the poles (in the ideal case). Such a piece is said to have a longitudinal field, or to belongitudinally magnetized.Figure 3-5. Horseshoe Magnet Straightened to Form a Bar Magnet.3.1.6.2.2A transverse slot or discontinuity in the bar magnet that crosses the magnetic flux lines will create north and southpoles on either side of the discontinuity (see Figure 3-6). The resulting leakage field will attract magnetic particles. Ina similar manner, a crack, even though it is very fine, will create magnetic poles as indicated in Figure 3-7. Thesepoles will also produce a leakage field that can attract magnetic particles. The strength of the leakage field will be afunction of the number of flux lines (i.e. the strength of the internal field), the depth of the crack, and the width of theair gap at the surface. The strength of this leakage field, in part, determines the number of magnetic particles that willbe gathered to form indications. Clear indications are found at strong leakage fields, while weak indications are formedat weak leakage fields.Figure 3-6. Slot in Bar Magnet Attracting Magnetic Particles.Figure 3-7. Crack in Bar Magnet Attracting Magnetic Particles.
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