An electron is a kind of elementary particle with a certain mass and negative charge. It can perform two kinds of motion; one is to move on the orbit around the nucleus, and the other is to spin on an axis passing through its center. Since the movement of electrons produces moments, currents and magnetic moments are produced during movement. In the applied constant magnetic field H, the magnetic moment of the electron acts like a small magnetic rod or needle. Since the spin quantum number of the electron is 1/2, the electron has only two orientations in the external magnetic field: one is parallel to H, corresponding to low energy Level, the energy is -1/2gβH; one is antiparallel to H, corresponding to the high energy level, the energy is +1/2gβH, and the energy difference between the two levels is gβH. If in the direction perpendicular to H, the electromagnetic wave of frequency v is added to meet the condition of hv=gβH, the low-energy level electrons absorb the electromagnetic wave energy and jump to the higher energy level, which is called electronic paramagnetic resonance.