A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.
In a rotating machine, the field coils are wound on an iron magnetic core which guides the magnetic field lines. The magnetic core is in two parts; a stator which is stationary, and a rotor, which rotates within it. The magnetic field lines pass in a continuous loop or magnetic circuit from the stator through the rotor and back through the stator again. The field coils may be on the stator or on the rotor.
The magnetic path is characterized by poles, locations at equal angles around the rotor at which the magnetic field lines pass from stator to rotor or vice versa. The stator (and rotor) are classified by the number of poles they have. Most arrangements use one field coil per pole. Some older or simpler arrangements use a single field coil with a pole at each end.
Although field coils are most commonly found in rotating machines, they are also used, although not always with the same terminology, in many other electromagnetic machines. These include simple electromagnets through to complex lab instruments such as mass spectrometers and NMR machines.