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
Search coil flux linked with the coil changes. This induces a current in the coil which can be registered on the galvanometer. Since induced current is directly proportional to rate of change of flux linkage and assuming the coil is removed from the magnetic field very quickly, the maximum current measured by the ammeter is proportional to the magnetic field formula_12. The search coil can be calibrated by repeating this in a known magnetic field. A reference table shows open coil voltage versus magnetic field A/m (or teslas) or short current versus magnetic field. Spacecraft with on board search coil magnetometers have