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
Induction coil radio transmitters, arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction. An induction coil consists of two coils of insulated wire wound around a common iron core "(M)". One coil, called the "primary winding" "(P)", is made from relatively few (tens or hundreds) turns of coarse wire. The other coil, the "secondary winding," "(S)" typically consists of many (thousands) turns of fine wire. An electric current is passed through the primary, creating a magnetic field.