Historically, electricity and magnetism were discovered and studied separately. A long time ago, Thales, an ancient Greek scientist, made a series of observations on static electricity. Based on these observations, he thought that friction magnetized amber. This is very different from the properties of magnetite and other minerals; Magnetite has natural magnetism. Magnets were first discovered in China, so ancient scientists in China invented Sina and compass.

Later, the connection between electricity and magnetism was discovered. For example, H.C. Oster of Denmark discovered the magnetic effect of current, and Ampere of France discovered the law of interaction between current and current. Later, Faraday put forward the law of electromagnetic induction, which integrated electricity and magnetism.

/kloc-In the middle of the 9th century, Maxwell put forward the unified electromagnetic field theory and realized the second comprehensive physics. The laws of electromagnetism and mechanics are completely different. According to Newton's idea, the interaction considered by mechanics, especially the interaction of universal gravitation, is a kind of interaction at a distance, and there is no problem of force transmission (of course, from a modern point of view, gravity should also have the problem of transmission), while electromagnetic interaction is the interaction of fields. From the distance action of particles to the "field interaction" of electromagnetic field, the concept has changed greatly. The effect of the field is highlighted.

The constant interaction between electric field and magnetic field causes the propagation of electromagnetic waves, which Hertz confirmed in the laboratory. Electromagnetic waves include not only radio waves, but also a wide spectrum, of which a very important part is light waves. In the past, the development of optics was completely independent of electromagnetism. After Maxwell's electromagnetic theory was established, optics became a branch of electromagnetism, and electricity, magnetism and light were unified.

This unification is of great technical significance. Almost all generators and motors are based on electromagnetic induction. The application of electromagnetic waves has led to modern radio technology. Until now, electromagnetism still plays a leading role in technology, so electromagnetism has always maintained an important position in basic physics.

Electromagnetism involves the electrodynamics of looking at problems and moving conductors in what reference system. Intuitively speaking, "current is a magnetic effect produced by charge flow", but judging whether charge flows or not involves the observer's problem-the reference system problem. Optics is a part of electromagnetism, so this problem can also be expressed as "What is the relationship between the propagation of light and the reference system". Michelson-Morey experiment shows that the speed of light in vacuum is constant in the inertial system. This confirms that electromagnetism follows the same law in the inertial system. This actually led to Einstein's special theory of relativity. Special relativity is basically the further development and popularization of electromagnetism. Michelson-Morey experiment has not been clearly explained in19th century, which is an important problem left over from19th century.