Magnetic field lines form in concentric circles around a cylindrical current-carrying conductor, such as a length of wire. The direction of such a magnetic field can be determined by using the "right hand grip rule" (see figure at right). The strength of the magnetic field decreases with distance from the wire. (For an infinite length wire the strength is inversely proportional to the distance.) Bending a current-carrying wire into a loop concentrates the magnetic field inside the loop while weakening it outside. Bending a wire into multiple closely spaced loops to form a coil or "solenoid" enhances this effect. A device so
Magnetic field the area times the magnetic field normal to that area. (This definition of magnetic flux is why is often referred to as "magnetic flux density".) The negative sign represents the fact that any current generated by a changing magnetic field in a coil produces a magnetic field that "opposes" the "change" in the magnetic field that induced it. This phenomenon is known as Lenz's law. This integral formulation of Faraday's law can be converted into a differential form, which applies under slightly different conditions. This form is covered as one of Maxwell's equations below. Similar to the way that a