A lightbulb (a plain old incandescent lamp) is typically brought up as an example of resistive load.

Yet the filament is actually made of several feet of very thin wire cleverly coiled to form a filament which is about one inch long. Clearly coiling the wire this way makes the filament more or less similar to an inductor. Yet lightbulbs are not considered inductive load.

Why are lightbulbs considered resistive load with a filament made of coiled wire?

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    \$\begingroup\$ I would guess that the filament resistance is much greater than the reactance at 50-60Hz. \$\endgroup\$
    – Armandas
    Commented Sep 18, 2012 at 11:31
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    \$\begingroup\$ Try to use one as a dummy load for a radio transmitter, and you will find plenty of evidence that it is inductive. But at line frequency, this is insignificant. \$\endgroup\$ Commented Sep 21, 2012 at 2:44

3 Answers 3


The reactance at 50 or 60 Hz is simply negligible. According to this calculator a coil of 100 turns 1 mm diameter, 25 mm long is 0.04 µH. At 50 Hz that's 12 µΩ. A 60 W bulb at 230 V has a resistance of 880 Ω, then the 12 µΩ is only 0.014 ppm of that, and the cos(φ) = 0.999999999999999902. Shall we say "1"?

The filament isn't coiled to add reactance, but to increase the resistance, by making the filament longer. Higher power bulbs don't need the higher resistance, and therefore often don't have a coiled filament. The alternative for increasing resistance would be to decrease the filament's diameter, but then it would become too fragile.

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    \$\begingroup\$ I often wonder if the coiled nature of the filament also helps with keeping the thing from breaking due to thermal, vibration and shock stresses?? Never ever saw any data to confirm that suspicion or not. \$\endgroup\$ Commented Sep 18, 2012 at 16:13
  • \$\begingroup\$ @Michael - Sorry, I don't know either. It would be nice if somebody can comment on this. \$\endgroup\$
    – stevenvh
    Commented Sep 18, 2012 at 16:19
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    \$\begingroup\$ @Michael: That's also a reason, it provides elasticity to reduce the shock due to stiffness, but they're also helped by the "third leg" (4th, 5th...) which help damping the new vibrations due to the elasticity (I'm sorry, I can't find better words right now). It helps, but it's not a decisive factor, see for example really small bulbs, where the risk of vibration is reduced due to the physical smaller geometry. Some are made with a considerable internal space even if they're half-a-centimeter, so the coil isn't for giving it a larger resistance, only. \$\endgroup\$
    – Vlad
    Commented Sep 19, 2012 at 7:22
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    \$\begingroup\$ @MichaelKaras: I see what you mean, but the coiled wire weighs more, and therefore under shock or vibration stronger forced will be applied to it. \$\endgroup\$ Commented Dec 11, 2013 at 17:02
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    \$\begingroup\$ The filament is, as I recall, typically a "second-order" coil--fine wire is coiled tightly, and then that coil is itself coiled into a spiral. You can sort of see that construction in the image above, actually, if you look closely (try looking at the bit that's not glowing on the far right). \$\endgroup\$
    – Hearth
    Commented Jul 25, 2021 at 3:39

coiling does not add resistance,(it adds reactance), it helps to place long filament in small space thus making light bulb small enough to be usable and thanks to small distance between windings they mutually heat each other thus increasing temperature of the filament and light output.Also it eliminate stress of elongating/shortening the filament while heating/cooling and from shocks and vibrations thus extending life span of light bulb.

  • \$\begingroup\$ The coiled wire weighs more, and therefore under shock or vibration stronger forced will be applied to it. So I'm not sure this will extend life span. \$\endgroup\$ Commented Dec 11, 2013 at 17:05

A slightly different way to look at this is to compare a light bulb "coil" to the coil of what are normally called inductive loads. If you take apart a motor or solenoid, you will see several orders of magnitude more turns with loops areas that are also several orders of magnitude bigger than the lamp filament.


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