Biot-Savart law (J.B. Biot, F. Savart) - The law which explains the contributions to the magnetic field by an electric current. This is analogous to the Coulomb's law. Mathematically: dB = (mu0 I)/(4 pi r2) dl cross e
Here dl is the infinitesimal directed length of an electric current causing the magnetic field, I is the current running via that directed length, r is the distance from that directed length, and e is the unit vector directed from the test point to the current-generating length.