The principle of the generator is the electromagnetic induction phenomenon discovered by Faraday. In 1820, Oster of Denmark discovered the magnetic field of current, revealing the relationship between electricity and magnetism. Scientists thought in reverse, since electricity can generate magnetism, can magnetism generate electricity? Many * scientists made unremitting efforts. Until 1831, the British physicist Faraday first made a breakthrough. Faraday found that when a part of the conductor of a closed circuit cuts the magnetic induction line in the magnetic field, an electric current will be generated in the conductor. This phenomenon is called electromagnetic induction. The generated current is called induced current. Faraday's discovery further revealed the connection between electricity and magnetism, led to the invention of generator, made the large-scale production and utilization of electric energy possible, and opened up the era of electricity.

According to the principle of the generator, the generator must have magnets and coils to transmit the electric energy generated by the generator, as well as copper slip rings and brushes. The following figure is the model of the generator. The coil rotates counterclockwise. In the half cycle from figure a to figure B to figure C, the AB side of the coil cuts the magnetic induction line downward, the induced current direction is from B to a, and the CD side cuts the magnetic induction line upward, the induced current direction is from D to C, the induced current direction in the coil is from D → C - + B - + A, and the current in the coil flows outward through the copper slip ring and brush, And the current direction of the external circuit flows from brush a to brush B. In the half cycle from figure C to figure J to figure a, the AB side of the coil cuts the magnetic induction line upward, the induced current direction is from a to B, and the CD side cuts the magnetic induction line downward, the induced current direction is from C to D, the induced current direction in the line is from a - → B - + C - + D, the current in the coil flows to the external circuit through the copper slip ring and brush, and the current direction of the external circuit flows from brush B to brush a.

Then the coil continues to rotate and the above process will be repeated. Therefore, the magnitude and direction of the induced current generated in the coil and the current flowing through the external circuit change periodically. This current is called alternating current. Such a generator is called an alternator. If the commutator (two copper half rings) is used to replace the copper slip ring of the alternator, although AC is generated in the line, DC is supplied to the external circuit (the direction remains unchanged). Such a generator is a DC generator.