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
Superconducting magnetic energy storage mean radius of coil is "R". "a" and "b" are width and depth of the conductor. "f" is called form function which is different for different shapes of coil. "ξ" (xi) and "δ" (delta) are two parameters to characterize the dimensions of the coil. We can therefore write the magnetic energy stored in such a cylindrical coil as shown below. This energy is a function of coil dimensions, number of turns and carrying current. Where Besides the properties of the wire, the configuration of the coil itself is an important issue from a mechanical engineering aspect. There are three factors