A photo of an electronic choke. 7 turns of enamelled wire wound on a ferrite rod. A through-hole component, mounted on a PCB (on the left of the photo there is a large electrolytic capacitor). The black mark at the top of the choke is adhesive, which fixes the ferrite core and the wire together for purely mechanical reasons.
The magnetic field lines have to close on the outside of the coil through air. Therefore, the size of the air gap is comparable to the length of the core.
choke_magnetica.jpg
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S. Zurek, Encyclopedia Magnetica, CC-BY-4.0
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