A generator is a device that converts mechanical energy into electrical energy.
The layout of a simple AC generator is shown in the figure below. The conductor is formed of a coil of wire, placed inside a magnetic field. The conductor is manually rotated within the magnetic field. This generates an alternating emf. The alternating current needs to be transmitted from the conductor to the load, which is the system requiring the electrical energy to function.
The load and the conductor are connected by a slip ring. A slip ring is a connector which is able to transmit electricity between rotating portions of a machine. It is made up of a ring and brushes, one of which is stationary with respect to the other. Here, the ring attaches to the conductor and the brushes are attached to the load. Current is generated in the rotating conductor, passes into the slip rings, which rotate against the brushes. The current is transmitted through the brushes into the load, and the system is thus powered.
The direction of the current changes with every half turn of the coil. As one side of the loop moves to the other pole of the magnetic field, the current in the loop changes direction. This type of current which changes direction is known as alternating current and the figure below shows how it comes about as the conductor rotates.
AC generators are also known as alternators. They are found in motor cars to charge the car battery.