Editing Magnetic core (section)

== Commonly used structures ==
=== Straight cylindrical rod ===
[[File:Ferrite-R-Core_(7).jpg|thumb|Ferrite rods are simple cylinders of ferrite that can be wound around.]]
Most commonly made of [[Allotropes of iron|ferrite]] or powdered iron, and used in [[radio]]s especially for tuning an [[inductor]]. The coil is wound around the rod, or a coil form with the rod inside.  Moving the rod in or out of the coil changes the flux through the coil, and can be used to adjust the [[inductance]]. Often the rod is [[screw thread|thread]]ed to allow adjustment with a screwdriver. In radio circuits, a blob of [[wax]] or [[resin]] is used once the inductor has been tuned to prevent the core from moving.

The presence of the high permeability core increases the [[inductance]], but the magnetic field lines must still pass through the [[air]] from one end of the rod to the other. The air path ensures that the inductor remains [[linear]]. In this type of inductor [[radiation]] occurs at the end of the rod and [[electromagnetic interference]] may be a problem in some circumstances.

=== Single "I" core ===

Like a cylindrical rod but is square, rarely used on its own.
This type of core is most likely to be found in car ignition coils.

=== "C" or "U" core ===
''U'' and ''C''-shaped cores are used with ''I'' or another ''C'' or ''U'' core to make a square closed core, the simplest closed core shape.  Windings may be put on one or both legs of the core.
{{multiple image
| align     = center
| direction = horizontal
| width     = 200
<!-- Image 1 -->
| image1    = U_core.png
| caption1  = A U-shaped core, with sharp corners
<!-- Image 2 -->
| image2    = C_core.png
| caption2  = The C-shaped core, with rounded corners
}}

=== "E" core ===
E-shaped core are more symmetric solutions to form a closed magnetic system. Most of the time, the electric circuit is wound around the center leg, whose section area is twice that of each individual outer leg. In 3-phase transformer cores, the legs are of equal size, and all three legs are wound.
{{multiple image
| align     = center
| direction = horizontal
| width     = 200
<!-- Image 1 -->
| image1    = E_core.png
| caption1  = Classical ''E'' core
<!-- Image 2 -->
| image2    = EFD_core.png
| caption2  = The ''EFD''' core allows for construction of inductors or transformers with a lower profile
<!-- Image 3 -->
| image3    = ER_core.png
| caption3  = The ''ETD'' core has a cylindrical central leg.
<!-- Image 4 -->
| image4    = EP_core.png
| caption4  = The ''EP'' core is halfway between a ''E'' and a ''pot'' core
}}

==== "E" and "I" core ====

Sheets of suitable iron stamped out in shapes like the ([[sans-serif]]) [[Letter (alphabet)|letters]] "E" and "I", are stacked with the "I" against the open end of the "E" to form a 3-legged structure. Coils can be wound around any leg, but usually the center leg is used. This type of core is frequently used for power transformers, autotransformers, and inductors.

<!-- diagram would be good here-->
{{multiple image
| align     = center
| direction = horizontal
| width     = 200
<!-- Image 1 -->
| image1    = ER core assembly.png
| caption1  = Construction of an inductor using two ''ER'' cores, a plastic bobbin and two clips. The bobbin has pins to be soldered to a [[printed circuit board]].
<!-- Image 2 -->
| image2    = ER core assembly exploded.png
| caption2  = [[Exploded view]] of the previous figure showing the structure
}}

==== Pair of "E" cores ====

Again used for iron cores. Similar to using an "E" and "I" together, a pair of "E" cores will accommodate a larger coil former and can produce a larger [[inductor]] or [[transformer]]. If an air gap is required, the centre leg of the "E" is shortened so that the air gap sits in the middle of the coil to minimize [[magnetic fringing|fringing]] and reduce [[electromagnetic interference]].

=== Planar core ===

A planar core consists of two flat pieces of magnetic material, one above and one below the coil. It is typically used with a flat coil that is part of a [[printed circuit board]]. This design is excellent for [[mass production]] and allows a high [[power (physics)|power]], small [[volume]] [[transformer]] to be constructed for low cost. It is not as ideal as either a '''pot core''' or '''toroidal core'''{{Citation needed|date=June 2010}} but costs less to produce.
{{multiple image
| align     = center
| direction = horizontal
| width     = 200
<!-- Image 1 -->
| image1    = Planar.png
| caption1  = A planar 'E' core
<!-- Image 2 -->
| image2    = Planar core assembly.png
| caption2  = A planar inductor
<!-- Image 3 -->
| image3    = Planar core assembly exploded.png
| caption3  = Exploded view that shows the spiral track made directly on the printed circuit board
}}

{{clear}}

=== Pot core ===

Usually ferrite or similar. This is used for [[inductor]]s and [[transformer]]s. The shape of a pot core is round with an internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit together around a coil former ([[bobbin]]). This design of core has a [[Electromagnetic shielding|shield]]ing effect, preventing [[radiation]] and reducing [[electromagnetic interference]].{{multiple image
| align     = center
| direction = horizontal
| caption_align = center
| image1    = RM core.png
| width1 = 200
| caption1  = A pot core of 'RM' type
| image2    = POT CORE.jpg
| width2 = 320
| caption2  = A Regular Pot Core
}}

=== Toroidal core ===
{{Main article|Toroidal inductors and transformers}}

This design is based on a [[toroid]] (the same shape as a [[doughnut]]).  The coil is wound through the hole in the torus and around the outside.  An ideal coil is distributed evenly all around the circumference of the torus.  The [[symmetry]] of this geometry creates a [[magnetic field]] of circular loops inside the core, and the lack of sharp bends will constrain virtually all of the field to the core material.  This not only makes a highly [[Energy conversion efficiency|efficient]] [[transformer]], but also reduces the [[electromagnetic interference]] radiated by the coil.

[[Image:Toroid core.png|thumb|150px|A toroidal core]]

It is popular for applications where the desirable features are: high [[Power-to-weight ratio|specific power]] per mass and [[volume]], low [[mains hum]], and minimal [[electromagnetic interference]].  One such application is the [[power supply]] for a hi-fi [[audio amplifier]].  The main drawback that limits their use for general purpose applications is the inherent difficulty of winding wire through the center of a torus.

Unlike a split core (a core made of two elements, like a pair of ''E'' cores), specialized machinery is required for automated winding of a toroidal core.  Toroids have less audible noise, such as mains hum, because the magnetic forces do not exert [[bending moment]] on the core.  The core is only in compression or tension, and the circular shape is more stable mechanically.

=== Ring or bead ===
{{Main article|Ferrite bead}}

[[File:Two inductors (437342545).jpg|thumb|On the left, a non-adjustable [[ferrite (magnetic core)|ferrite]] rod with connection wires glued to the ends. On the right, a molded ferrite rod with holes, with a single wire threaded through the holes.]]
[[Image:Ferrite bead no shell.jpg|thumb|right|A ferrite ring on a computer data cable.]]
The ring is essentially identical in shape and performance to the toroid, except that inductors commonly pass only through the center of the core, without wrapping around the core multiple times.

The ring core may also be composed of two separate C-shaped hemispheres secured together within a plastic shell, permitting it to be placed on finished cables with large connectors already installed, that would prevent threading the cable through the small inner diameter of a solid ring.