T.O. 33B-1-1
3.4.3
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 of
the U. When electrical current is passing through the coil, the two ends of the core are magnetized with
opposite polarity and the combination is an electromagnet similar to a permanent horseshoe magnet.
3.4.3.2
Fixed Leg Yoke.
A fixed leg yoke is shown in Figure 3-29. The legs are spaced approximately 5 inches apart providing a
usable magnetic field of approximately 25 in2. Fixed leg probes can be used on f lat, contoured, or irregular
surfaces. However, the fixed leg position might preclude their use on some parts of a complex configuration
unless 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)
3.4.3.3
Articulated Leg Yoke.
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 3-30
shows articulated-leg yokes with the legs in normal, decreased, and extended positions. The legs may be
moved to the decreased or extended positions to obtain optimum contact assuring a better induced magnetic
field. When in the extended position, the area of the usable magnetic field is increased though the field
strength is weaker. The discontinuities being sought must be larger. When in the decreased position, the
area of the usable magnetic field is decreased and the magnetic field is increased, permitting the detection of
finer discontinuities. Movable-leg yokes are more suitable for demagnetization than fixed-leg yokes. The
space between the poles or legs can be adjusted such that the parts to be demagnetized pass snugly between
them to obtain maximum demagnetization.
3-50
Change 2
