As early as 1820, the Danish scientist Auster discovered the magnetic effect of currents, and for the first time revealed the existence of a link between magnetism and electricity, thus linking electricity with magnetism.
In order to explain the phenomenon of permanent magnets and magnetization, Ampere proposed a molecular current hypothesis. Ampere believes that there is a circular current in any material molecule, called the molecular current, and the molecular current is equivalent to a primitive magnet. When the material does not exist in the macroscopic magnetic, the orientation of these molecular currents is irregular, and their magnetic effects on the outside cancel each other, so that the whole object is not magnetic. Under the action of an external magnetic field, each molecular current equivalent to the elementary magnet will tend to be oriented in the direction of the external magnetic field, and the object will be rendered magnetic.
There is an essential link between magnetic phenomena and electrical phenomena. The magnetic properties of matter have a close relationship with the structure of movement of electrons. The concept of electron spin first proposed by Uhlenbeck and Goldschmidt is to regard electrons as a charged ball. They think that, similar to the movement of the earth around the sun, electrons operate on the one hand around the nucleus and have corresponding orbits. The angular and orbital magnetic moments, on the other hand, spin around their own axis, with spin angular momentum and the corresponding spin magnetic moment. The magnetic moment measured by Stern-Gellach from the silver atom ray experiment is exactly this spin magnetic moment.