Biot and Savart's law The magnetic field dB due to a small current element idl (i.e. a small element of a conductor of length dl and carrying a current i) is given by,

dB = m ₀ i (dl) ´ r/(4p r²) [Weber/m² ]

where m ₀ = 4p ´ 10^-7 Weber/A.m is the permeability of free space. r is the distance between the current element and the point at which dB is being determined, and r the unit vector along r ( r = r/r).