Forces created by the presence of an electric current, and electric charge, or a magnet.

The existence of an electric field is made known by its effect on another electric charge, and the existence of a magnetic field can be made known by its effect on another magnet.

A field around a magnet or an electric current will deflect a small magnet; such as, a compass needle, in a particular direction when it is placed in such a field.

The direction in which the north pole of the magnet points is normally called the direction of the field and the direction of the field generally follows curved lines of force.

Four equations, formulated by James Clerk Maxwell, that together form a complete description of the production and interrelation of electric and magnetic fields.

The statements of these four equations are as follows:

1. Electric field diverges from electric charge.
2. There are no isolated magnetic poles.
3. Electric fields are produced by changing magnetic fields.
4. Circulating magnetic fields are produced by changing electric fields and by electric currents.

Maxwell based his description of electromagnetic fields on these four statements.