FIG. A-10. Ampere’s law. A current I through a length of wire ΔL creates at a point P a magnetic field ΔB given by ΔB=10–7ΔL X r)/r3, the magnitude of the field being ΔB=10–7(sinΔL/r2), where ΔB is in webers/m2 when I is in amperes and r and ΔL are in meters. The vector ΔL is in the direction of the current and r is the vector from the element ΔL to P.