本文从两个载流线圈之间的能量的纽曼公式推导了准静态电磁场方程，然后讨论了3种方法从准静态方程过度到辐射电磁场方程。1）Maxwell提出的位移电流方法，2）洛伦兹的滞后势方法，3）作者提出的基于能量守恒的包含超前波的方法。作者分析了磁场的不同定义，按照安培力，洛伦兹力，矢量势的旋度，滞后矢量势的旋度，法拉第电磁感应，坡印廷矢量，互能流定义的磁场。磁场定义还分在环路上和在直线上求平均值两种不同类型定义方法。在这些不同方法中，作者认为用互能流定义的磁场是正确的。按照这个磁场定义电磁波的磁场和电场保持90度相位差。不是像麦克斯韦电磁理论那样磁场和电场保持同相位。这样电磁波的确是无功功率的波。这种波的能量是不会溢出宇宙的。单独靠电磁波并不能传递能量。传递能量得靠互能流。互能流由同步的滞后波和超前波构成。互能流具有光子的性质，作者认为互能流就是光子。本文的重点是由互能流定义了磁场。由于互能流不管是在准静态条件下，按照麦克斯韦电磁理论，还是按照作者的电磁理论都成立。因此这样定义的磁场是合理的。这个定义和麦克斯韦按照磁矢量势定义磁场是不相同的。

This article derives the quasi-static electromagnetic field equation from the Newman formula of the energy between two current-carrying coils, and then discusses three methods to transition from the quasi-static equation to the radiation electromagnetic field equation. 1) The displacement current method proposed by Maxwell, 2) Lorentz's hysteresis potential method, 3) The method based on energy conservation and including advanced waves proposed by the author. The author analyzes different definitions of magnetic fields, including magnetic fields defined in terms of Ampere force, Lorentz force, curl of vector potential, curl of hysteresis vector potential, Faraday electromagnetic induction, Poynting vector, and mutual energy flow. The definition of magnetic field is also divided into two different types of definition methods: averaging on a loop and averaging on a straight line. Of these different approaches, the authors consider the magnetic field defined in terms of mutual energy flow to be correct. According to this magnetic field definition, the magnetic field and electric field of electromagnetic waves maintain a phase difference of 90 degrees. It is not like Maxwell's electromagnetic theory that the magnetic field and the electric field remain in the same phase. In this way, electromagnetic waves are indeed waves of reactive power. The energy of this wave will not overflow into the universe. Electromagnetic waves alone cannot transmit energy. The transfer of energy depends on mutual energy flow. The mutual energy flow consists of synchronized delayed waves and advanced waves. Mutual energy flow has the properties of photons, and the author believes that mutual energy flow is photon. The focus of this article is on the magnetic field defined by mutual energy flow. Since the mutual energy flow is established no matter under quasi-static conditions, according to Maxwell's electromagnetic theory or according to the author's electromagnetic theory. Therefore the magnetic field defined in this way is reasonable. This definition is different from Maxwell's definition of magnetic field in terms of magnetic vector potential.