14.3 The equivalent Volume Current Density Je One molecule has a current I with an area a, so its magnetic dipole moment is m=lae Consider a small length dl Taking dl as the hight and a cos o as the cross sectional area, we get a small volume adl cos o a molecule in this volume will contribute a current ladl cos 6 All the molecules in this volume contribute a current N/ adl cos0=Nm·dl=M·dl