T.O. 33B-1-13-37A = p(6)^{2}A=p(1)^{2}A = 113 sq. inches A = 3.14 sq. inches3.3.12.6.2.1When the cross-sectional area of the part is less than one-tenth of the cross-sectional area of the coil, the part should bemagnetized lying in the bottom of the coil.3.3.12.6.2.2When the cross-sectional area of the part is greater than one-tenth of the cross-sectional area of the coil, the part mustbe magnetized in the center of the coil.3.3.12.6.2.3When using a cable wrap or when the cross-sectional area of the part exceeds one-half of the cross-sectional area of thecoil, the part should be centered in the coil and the formula for high fill factor coils SHALL be used for estimating therequired amperage.3.3.12.6.2.4Table 3-1 lists the diameter of the largest part that can be magnetized lying in the bottom of a coil or placed next to thecoil wall for some typical coil sizes. For any given coil diameter, parts with diameters larger than those listed must bemagnetized by some other method, such as centering them in the coil, using a cable wrap, or using a larger coil.Table 3-1. Coil Size vs. Maximum Diameter for Parts Magnetized in Bottom of Coil.Coil Diameter(inches)Maximum PartDiameter (inches)8 2.512 3.815 4.818 5.720 6.324 7.63.3.12.6.3CalculatingCoilCurrent.Two rule-of-thumb formulas have been developed for use in estimating the coil current levels to be used forlongitudinal magnetization. One formula is for a part centered in the coil and the other for a part lying in the bottom ofthe coil. These formulas apply to cylindrical and irregularly shaped parts and at one time were thought to estimate therequired current to within 10%. Recent studies show that in almost all instances they overestimate the required currentby at least 50%. They use the part length-to-diameter (L/D) ratio. The useful magnetizing field produced by anencircling coil extends approximately 6 to 9 inches to either side of the coil. For parts longer than the effective fielddistance, one or more inspections are required along the length of the part. When repositioning these longer parts inthe coil, allow a 3-inch effective field overlap. The formulas are intended for part with a L/D ratio between 3, and 15.For inspecting parts having L/D ratios of 3 or less, see paragraph 3.3.12.6.3.7. For parts with a L/D ratio greater than15, use 15 as the value for the ratio.3.3.12.6.3.1 FormulaforPartLyinginBottomofCoil.The following formula can be used when the cross-sectional area of the part is less than one-tenth the cross-sectionalarea of the coil(s) and SHALL be used whenever the part is lying in the bottom of the coil, or is placed next to the coilwall during magnetization. If the part has hollow portions, replace D with D_{eff} (see paragraph 3.3.12.6.3.4).IKDNL=

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