Magnetic field
![Magnetic field of an ideal cylindrical magnet with its axis of symmetry inside the image plane. The magnetic field is represented by magnetic field lines, which show the direction of the field at different points.](/Images/godic/202412/21/VFPt_cylindrical_magnet_thumb.svg1440.png")
![One of the first drawings of a magnetic field, by René Descartes, 1644. It illustrated his theory that magnetism was caused by the circulation of tiny helical particles,](/Images/godic/202412/21/Descartes_magnetic_field1440.jpg")
![Hans Christian Ørsted, Der Geist in der Natur, 1854](/Images/godic/202412/21/lossy-page1-220px-Ørsted_-_ger,_1854_-_682714_F.tif1440.jpg")
![Compasses reveal the direction of the local magnetic field. As seen here, the magnetic field points towards a magnet's south pole and away from its north pole.](/Images/godic/202412/21/Magnetic_field_near_pole.svg1440.png")
A magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter (symbol: A·m or A/m) in the SI. B is measured in teslas (symbol:T) and newtons per meter per ampere (symbol: N·m·A or N/(m·A)) in the SI. B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges.