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
Helmholtz coil coil is "n" and the current through the coils is "I", then the magnetic field B at the midpoint between the coils will be given by where formula_15 is the permeability of free space . Start with the formula for the on-axis field due to a single wire loop which is itself derived from the Biot–Savart law: Here The Helmholtz coils consists of "n" turns of wire, so the equivalent current in a one-turn coil is "n" times the current "I" in the "n"-turn coil. Substituting "nI" for "I" in the above formula gives the field for an "n"-turn coil: