Abstract: According to the hypothesis of Amber's molecular electric current,the physical model of ideal magnetic pole is made by a thin solenoidal circuit,and the relation between the magnetic charge on the pole and the magnetic moment per unit length of the thin solenoidal circuit is got.It is verified that Coulomb's magnetic law establishes only to straight thin solenoidal circuits.Imitating the physics model of ideal magnetic pole,magnetized needles are modeled on many thin soleniodal circuits of molecular electric current.The establishing condition of Coulomb's magnetic law is got.