(1) Ampere's Circuital Law relates the net magnetic field along a closed loop to the electric current passing through the loop. It was first discovered by André-Marie Ampère in 1826. (2) The law states that the line integral of the magnetic field over a closed path equals the permeability of free space times the total current enclosed by that closed loop. It can be expressed mathematically as an integral or differential equation. (3) Applications of the law include calculating the magnetic field inside and outside a long cylindrical conductor, finding the magnetic field of a solenoid, and determining the magnetic field of a toroidal solenoid.