By an analytic method, in this essay a vector invariant of a casual point to a tetrahedron on volume and deductions form vector invariant are given. 摘要利用解析法，给出空间任意一点与四面体体积有关的一个向量恒等式，并给出若干推论。
At the airgap, magnetic field change is analysed through two-dimensional analytical method and two-dimensional finite-element method. 通过二维解析法和有限元法分析电动机不同气隙时的气隙磁场变化。
