Table 3-3 compares the differences in the current required to longitudinally magnetize the solid and hollow parts in the
examples of paragraphs 184.108.40.206.3.1 and 220.127.116.11.3.4 above. The only difference in the two parts is that one was
hollow and the other was solid. If the effective diameter Deff had not been considered, the current for the hollow part
would have been over-estimated by 927 amperes. This additional amperage would certainly result in excessive
background and possibly false indications from over-magnetizing the part.
Table 3-3. Comparison of Coil Amperages for Solid vs. Hollow Parts
The inspector should be cautioned that when using the above rule-of-thumb formulas, the part length used in the L/D
ratio is the part dimension measured in the direction of the coil axis and the diameter is the dimension measured in the
plane of the coil. For example, a 2-inch diameter steel bar 10 inches long will have an L/D ratio of 5 when the bar is
placed in the coil with its axis parallel with that of the coil. If the bar is placed in the coil so that the bar and coil axis
are at right angles to each other, the L/D ratio will be only 0.2, a figure which, if used, would indicate the need for
impracticably high amperages.
If the need arises to inspect parts having L/D ratios of 3 or less, the effective L/D ratio must be increased by placing the
part between two pole pieces while it is being magnetized. The length dimension for the L/D ratio then becomes the
length of the two pole pieces plus the part length. Such pole pieces must make good contact on each side of the part
and must be made of ferromagnetic material. Solid steel pole pieces may be used when direct current is used in the coil
and the continuous method of inspection is used. If the continuous method is used with either AC or half-wave DC
current in the coil, the pole pieces must be made from laminated magnetic material similar to the silicon steel legs of a
hand probe with articulated legs. This is also true for residual inspection. Pole pieces must be made from the proper
material if residual inspection, or the wet continuous method of inspection with AC or half-wave DC, is to be used.
Cable Wrap Coil.
Cables used are commonly 2/0 or 4/0 AWG (American Wire Gage), flexible stranded, insulated copper cable. The
number of turns used is kept low, from 3 to 5 turns to minimize cable resistance in the case of DC and coil impedance
when AC is used.
Multiple inspections spaced approximately 15 to 18 inches along the length of a long part are preferable to one
inspection using one long coil of many turns. Cable lead lengths between the power source and coil wraps must be kept
as short as practical so that maximum amperages are produced in the coil. When AC is being used, and to some extent
with half-wave DC, available amperages can be increased by twisting or taping together the cable lengths between the
coil and the power supply. This reduces the coil-circuit impedance the same way that reducing turns on the coil does
and makes it possible for more AC current to flow in the coil circuit. The total length of the cable together with the
resistance of its connections determines the DC amperage obtainable in the coil. The longer the cable and the poorer
the electrical connections, the less will be the DC and the half-wave DC amperages that can be obtained. Increased
cable resistance also lowers available AC current, but in the case of AC, the impedance of the coil and coil length
circuit has a much greater effect than does resistance in lowering and limiting available AC current.