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
Field-reversed configuration such experiments, as above, a cylindrical coil produces a uniform axial magnetic field and gas is introduced and ionized, creating a background plasma. Neutral particles are then injected into the plasma. They ionize and the heavier, positively-charged particles form a current ring which reverses the magnetic field. Spheromaks are FRC-like configurations with finite toroidal magnetic field. FRCs have been formed through the merging of spheromaks of opposite and canceling toroidal field. Rotating magnetic fields have also been used to drive current. In such experiments, as above, gas is ionized and an axial magnetic field is produced. A rotating magnetic field