Biot-Savart law (J.B. Biot, F. Savart)
A law defines the contributions to a magnetic field through an electric current. This is analogous to Coulomb's law. Mathematically specking, it is dB = (mu0 I)/(4 pi r2) dl cross e where dl refer to the infinitesimal directed length of the electric current causing by the magnetic field, I refer to the current passing through that directed length, r refer to the distance from that directed length, & e refer to the unit vector directed from the test point to current-generating length.