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 detector antenna ("A") is received by a tuner ("not shown") and passed through the excitation coil "C", the other end of which is connected to ground ("E"). The rapidly reversing magnetic field from the coil exceeds the coercivity "H" and cancels the hysteresis of the iron, causing the magnetization change to suddenly move up the wire to the center, between the magnets, where the field reverses. This had an effect similar to thrusting a magnet into the coil, causing the magnetic flux through the pickup coil "D" to change, inducing a current pulse in the pickup coil. The audio pickup coil