Biot-Savart's Law

Biot-Savart's Law states that the intensity of the magnetic field set up by a current flowing through a wire varies inversely with the distance from the wire. This principle is fundamental to modern electromagnetic theory. The Biot-Savart law is also used to calculate the velocity induced by vortex lines in aerodynamic theory.

For an vortex line of infinite length, the induced velocity at a point is given by

v = Γ/(4πd)

where Γ is the strength of the vortex, and d is the perpendicular distance between the point and the vortext line. For vortex segments of finite length, a similar expression holds, but it involves also the cosines of the sight angles from the point to the ends of the segment. (For those more interested in the electromagnetic applications of the law, vorticity is analagous to current, and induced velocity to magnetic field strength.)

See: Jean Baptiste Biot, magnetism, vorticity