Faraday conceived a force line image depicting electromagnetic action from extensive experimental research. He thinks that charge and magnetic pole are surrounded by magnetic field lines, and charge (or magnetic pole) is connected by magnetic field lines (including electric field lines and magnetic field lines).

Inspired by Faraday's idea of magnetic field lines, Maxwell published two papers on electromagnetic similarity in 1842 and 1847, which transformed Faraday's idea of magnetic field lines into quantitative expression and initially formed a unified theory of electromagnetic action.

Maxwell was deeply attracted by Faraday's electromagnetic thought when he was in college. He realized the importance of the concept of force line and the weakness of Faraday's qualitative expression, and decided to make up for it by mathematical means. At the same time, Thomson's paper made him realize that Faraday's thought is interlinked with the traditional electrostatic theory, and it is possible to further establish a unified electromagnetic theory.

1856 In February, Maxwell's first electromagnetic paper, On Faraday's magnetic field lines, not only explained Faraday's magnetic field lines in mathematical form, but also included all his later new ideas, even the embryonic form of Maxwell's equation.

Faraday proved that magnetic energy can generate electricity. Electric current is different from electric field. Current can obviously heat wires and electrolyze water, which is called conduction current. Although the changing electric field has some properties of current, it is not obvious. Smart Maxwell named it "displacement current".

Can a changing electric field excite a magnetic field like an electric current? Faraday has been experimenting for many years, but he still hasn't found the connection between them. At the most critical time, problems are often not solved by experiments, but can only be decided by reasoning. This problem was really deduced by Maxwell with mathematical formula.

1865, the unified electromagnetic field theory was finally born. Maxwell published a set of equations describing the law of electromagnetic field motion. He proved that changing magnetic field can produce electric field, and changing electric field can produce magnetic field. Faraday's prediction has been perfectly expounded and rigorously demonstrated mathematically. What's even better is that Maxwell used his own equation to deduce that the speed of electromagnetic waves is exactly equal to the speed of light, which proves that light is also an electromagnetic wave. Optics and electromagnetism merge here.