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 analogous to current, and induced velocity to magnetic field strength.)

See: Jean Baptiste Biot, magnetism, vorticity