Curved lines used to represent a magnetic field, drawn such that the number of lines is related to the strength of the magnetic field at a given point and the tangent of any curve at a particular point is along the direction of magnetic force at that point.磁力线：用来表示磁场的曲线，其数目与磁场中某一点的磁力有关，任何曲线在某一点上的切线方向就是这一点上的磁力方向