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Ok so I am learning about induction and I always see coils of wire with no insulator. So I was wondering, What is the significance of it being a coil of wire rather than just a solid cylinder since there are already many points of contact between each "wrap" of the coil?

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up vote 13 down vote accepted

The part you are missing is that what looks like uninsulated wire actually isn't. A lot of enamel coated or "magnet wire" can look like bare copper at first glance, but the wire is actually coated with a thin semi-transparent insulation layer. The reason for using thin insulation is so that lots of turns of the coil can fit into the tightest possible space.

Electrically, a coil of wire is quite different from a cylinder. To make a magnetic field, you need current flowing around where you want the field. Think of a cylinder of current surrounding the area. The magnetic field is proportional to the total current in the cylinder.

A coil is sortof a cheat on that. The same current is re-used each turn to add to the apparent cylinder current. Let's say you have a coil with 100 turns. If you run 1 A thru it, each of those turns contributes 1 A to the overall cylinder current. You get the same magnetic field as if it were a solid cylinder with 100 A running around it.

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Thanks so much for your answer! As a follow up to the question, is a cylinder going to be the optimal design for induction? might more of an oval shape be better so as to induce current over a longer length of the wire? – weezybizzle Jan 4 '12 at 19:20
@weezy: To make the strongest magnetic field, you want current as tightly wrapped around it as possible. Unless the shape or object you want to magnetize is oval, oval current shape around it will be less efficient than circular. There is no advantage to longer distance of current, only the total current going around. – Olin Lathrop Jan 4 '12 at 20:38
@weezybizzle: on the contrary: what counts for inductivity of a solenoid coil is not the length of the wire but the AREA enclosed by the wire. A circular shape gives you the largest area per given wire length (perimeter). – Curd Jan 4 '12 at 21:40
@curd if I have a wire 4pi in length, I can make one circle with radius 2 or 2 circles with radius 1, the former has twice the area of the latter. Does this mean I should make my coil as wide as possible to get the maximum induction? – spraff Jan 5 '12 at 9:39
@spraff: actually the former has four times the area of the latter (not just twice)! But both variants have the same inductivity, because not only area (=A) is a factor but also the square of the number (=N) of turns: L = mu * N^2 * A / l – Curd Jan 5 '12 at 9:50

Wire used in coils usually has an insulating coating - enamel or similar.

As long as that is intact, the coil does not approximate a solid conducting cylinder (which would be a single "turn" of very thick wire), but instead a multi-turn coil where the magnetic field sees the current multiplied by the number of turns and the inductance varies as the square of the number of turns.

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As the wire becomes thinner, resistance increases. Won't this dampen the benefits of the extra turns? Or does it cancel out somewhere? – spraff Jan 5 '12 at 9:42
@spraff - resistance can be an issue, yes, so the wire size is carefully chosen. At RF frequencies the skin effect additionally means that current only flows near the surface of the conductor, so physically large inductors in multi-kilowatt transmitters may even be wound with hollow tubing as their "wire". Also, the length of the coil along the core will reduce the inductance - so yes, there are a number of tradeoffs. – Chris Stratton Jan 5 '12 at 14:20
Litz wire is also used to mitigate the skin effect at high frequencies. This is many thin strands of wire individually insulated (enamelled) so the skin (for effect) is increased for the same amount of copper. Similar wire is used on contemporary ear-buds for flexibility and savings on insulation diameter with the various bundles colour coded with the enamel. en.wikipedia.org/wiki/Litz_wire – KalleMP Feb 11 '15 at 11:17
A (hollow) cylinder isn't really a 'turn' unless it's got a slit through one side – Nick T Feb 11 '15 at 20:49

The difference is that electrons follow the path of least resistance. For a solid cylindrical conductor that would be "straight through." For a coil of wire the current must flow "round and round" in space. The importance of this is related to what's going on physically here (ala Maxwell's Equations). Current flowing through a conductor induces a magnetic vector field that "wraps around" the direction the current is flowing in. The topology of a coil is such that these magnetic fields are superimposed so as to create a net magnetic field that has a kind of "flows out" of one end of the coil and "wraps around" back into the other end. Without going into a whole lot of math, it's this composite magnetic field that I believe gives an inductor the properties we are used to thinking about in circuits (i.e. resistance to changes in current). It's also this net magnetic field where the energy is effectively "stored" in an inductor.

Re-reading your question, it is essential that the windings of a coil not "touch" electrically or you will not get the effects I described above. They may in fact only "look" like they are not insulated, but in fact they have to be, as Chris pointed out.

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@OlinLathrop, can you explain the downvote please? – vicatcu Jan 4 '12 at 19:35
No, since I didn't downvote this. – Olin Lathrop Oct 18 '12 at 14:35

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