The electric current density vector field also satisfies superposition principle, some applications thereof are introduced. 摘要电流密度矢量场也满足叠加原理，给出了一些应用电流密度场叠加原理的例子。
We shall use various schemes, none of them wholly satisfactory, to depict vector fields in this book. 在本书中我将用不同的图样画出矢量场，但没有一种是完全令人满意的。
