T.O. 33B-1-13.4.3 Equipment.3.4.3.1 General.Yokes or probes are essentially U-shaped laminated cores of soft iron with a coil wound around the base ofthe U. When electrical current is passing through the coil, the two ends of the core are magnetized withopposite polarity and the combination is an electromagnet similar to a permanent horseshoe magnet.3.4.3.2FixedLegYoke.A fixed leg yoke is shown in Figure 3-29. The legs are spaced approximately 5 inches apart providing ausable magnetic field of approximately 25 in2. Fixed leg probes can be used on f lat, contoured, or irregularsurfaces. However, the fixed leg position might preclude their use on some parts of a complex configurationunless special pole pieces are available to adapt the legs to the part’s surface.Figure 3-30.Leg Positions of Articulated Leg Yoke (Sheet 1 of 3)3.4.3.3ArticulatedLegYoke.An articulated or movable-leg yoke (also shown in Figure 3-30) contains all the features of a fixed-leg yoke.They are, however, more versatile in their use and application because of the movable legs. Figure 3-30shows articulated-leg yokes with the legs in normal, decreased, and extended positions. The legs may bemoved to the decreased or extended positions to obtain optimum contact assuring a better induced magneticfield. When in the extended position, the area of the usable magnetic field is increased though the fieldstrength is weaker. The discontinuities being sought must be larger. When in the decreased position, thearea of the usable magnetic field is decreased and the magnetic field is increased, permitting the detection offiner discontinuities. Movable-leg yokes are more suitable for demagnetization than fixed-leg yokes. Thespace between the poles or legs can be adjusted such that the parts to be demagnetized pass snugly betweenthem to obtain maximum demagnetization.3-50Change 2
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