# Why are coils of wire used in heating vs a straight wire?

Attached to this you will find a picture of a bathroom heating element. I noticed that the wires that emit heat are coils. From what I understand this effectively makes them inductors. Why is an inductor preferred over a long straight wire?

• Filament lightbulbs have to fit even more wire into an even smaller space, so they use coiled coils. Jun 24, 2023 at 22:46
• Why is an inductor preferred? ... please do not make an assumption that inductance is the desired property Jun 24, 2023 at 23:24

Three benefits to coiled resistance wire used in heating:

1. Thermal expansion. There is no strain on the terminals.
2. Easier to tune the resistance to the required voltage and power requirements for almost any application.
3. Easier to distribute heat over a surface because of the coil diameter.

There are applications where straight wires are used. Bread toasters is an example.

You are correct in observing that the inductance of the coil increases. However a coil is not required for inductance. Only current is required. Coiling increases the inductance in a compact form.

In the case for resistance heaters, the resistance is the dominant parameter. When applying pulse width modulation, the inductive kick must be handled.

• This is a good answer, but I think your last sentence could do with some clarification as there are both resistive and inductive heaters. I believe that it's resistance that causes the heat in both cases, but in the case of inductive heating it's the resistance of the object in which the current is induced rather than in the coil itself. Jun 26, 2023 at 13:36
• Sure, but the OP is clearly confused about inductive vs resistive heating so I think it's worth the clarification. It's your answer, so it's up to you though. Jun 26, 2023 at 15:35
• Thermal expansion is a big point. When I had to make a linear wire heater, I had to add a spring to maintain tension as it heated. That wasn't even all that hot, but it did need decent tension (I needed a hot wire cutter to cut 150mm-thick foam in such a hurry I could only make it from my stash of junk) Jun 26, 2023 at 16:16
• I had forgotten about those cutters. I have seen stationary thermal wire cutters for foam board that use a counter weight on a polley system to maintain a precise tension You can use the position of the counter weight to measure temperature. @ChrisH. Jun 26, 2023 at 16:22
• @RussellH that's a neat idea. I used a tent pole spring on the other end of a lever, which along with the guide fence was made from scrapwood. But it wasn't table-mounted (a mattress would need a big table). Jun 26, 2023 at 18:43
• From what I understand this effectively makes them inductors.

Not at line frequencies. At 50 or 60Hz the contribution of their inductance to their behavior is so minuscule as to only be theoretical. Moreover, because it's resistance wire, at whatever radio frequencies that their inductance does become meaningful, they wouldn't be very high quality inductors.

Mostly, they're coiled for mechanical convenience, as mentioned in @RusselH's answer.

They are not inductors, they are just coiled so they can fit a longer wire. If it was straight from A to B it would draw too much current. They coil it so they can make them the correct resistance (and therefore power) for whatever power rating the element is designed for.

• Thinner wire could also reduce the current, but very thin wire is fragile. Jun 24, 2023 at 22:46
• A short length of straight wire would have a very small surface area. It would need to be heated to incandescence to radiate any significant heat, which would shorten its life and illuminate the room rather than heat it. Jun 25, 2023 at 22:31
• @Paul_Pedant but a light bulb filament is a coiled coil Jun 26, 2023 at 19:34
• @DDuck Sure, but the question is about a heating element, and that needs to output radiation in the infrared range. A lamp needs to emit mainly white light. For any resistive load, there is a whole balance between supply voltage, unit resistivity, cross-sectional area, length, and required temperature (and note the resistivity is extremely temperature-sensitive). Once you get that stuff optimally worked out, you fit the element in the available space by coiling it as much as you need. Inductance and capacitance have no significant effect here. Jun 27, 2023 at 8:23

A coiled wire makes things more thermally and mechanically stable. Under heat, wire tends to expand. A straight wire would become slack and in danger of touching the next wire. A coil under tension will be able to contain the slack. If one area of the coil gets hotter than another, it will become looser, and with the windings gaining distance, the temperature goes down due to less inter-coil heating.

As a rule, at mains frequency (and voltage), any significantly effective inductivity will require an iron-plated core; the "inductivity" of the depicted coils will become noticeable only at RF frequencies.

Langmuir in 1912 worked out that flow of gas over the filament cooled it, and that there was a stationary layer of gas surrounding it. (Langmuir sheath)

A coil has more filament inside the sheath and less convective cooling and glows hotter. The hotter filament is a better radiator of heat (Stefan-Boltzmann's law) and perhaps a better match to the bathroom type of heater shown in the OP's photo. A radiant heat source (over a convective) is also better for items such as toasters.

The heater coil is typically mounted horizontally so the amount of cooling is uniform over its length.

The amount of surface area per unit length of heater coil is lower than a straight heater element and requires less support points for the same radiant output.

• But coiled heater wires are also used in fan heaters and freestanding convection heaters, both of which rely on heating the air to heat the room. Heat guns and hair dryers too. Jun 27, 2023 at 14:07