The cross-sectional area of the part is not more than 1/10 the of the coil.
¾ The part is placed at the bottom of the coil.
¾ The permeability of the part is greater than (500 gauss/oersted) or 628.3 ´ 10-6
henry
(H)/metre (m).
¾ L/D ratio equal to or greater than 3.