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I was playing with a dimmer switch controlling an incandescent lightbulb and noticed that when the current was low enough, I could discern slight variability of brightness along the filament (something I wasn't able to notice at higher currents).

I've included two pictures showing the variability – the second one is a close-up of the junction whereby the filament wraps around the post and return upwards. One can clearly see that there practically is no brightness directly at said the junction.

entire filament close-up of the junction

I'm curious as to why the brightness of the filament dims near this junction – is the length that then passes over the post made of a different material, thus not glowing as intensely? I thought it might be so that the junction/filament junction does not heat up intensely, thereby losing structural integrity and making it more vulnerable to snapping after a sudden, violent impact.

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  • $\begingroup$ I wish you hadn't edited out your own theory about why this was happening; there's no shame in speculation. $\endgroup$ Commented Nov 30, 2017 at 16:10
  • $\begingroup$ I thought it might ease the reference of the question/answer in the future, but I'll gladly reinstate my original speculation. $\endgroup$
    – falsovuoto
    Commented Nov 30, 2017 at 16:27

2 Answers 2

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When a current flows through a wire, it heats up the wire continuously. This heat has to go somewhere. Some of the heat energy is lost to electromagnetic waves; most of these are infrared radiation, but a small fraction of these waves are what you perceive as light. The hotter the filament is, the more light it will emit. Eventually, as the bulb warms up, it'll get to a point where the rate at which heat is created by the current passing through it is equal to the rate at which heat is lost to radiation, and an equilibrium is reached.

However, the ends of the filaments are also in contact with the brackets that hold up the filaments. They can shed this heat not only by radiation, but also by conduction: a hotter object placed in contact with a colder object will lead to a heat flow from hot to cold. In particular, the filaments are in contact with the bracket wires, which are in contact with the glass ring in the center, which is in turn in contact with the base of the bulb and the lamp, etc.

This means that the ends of the filaments can lose the heat from the current a lot more efficiently than can the centers of the filaments, which in turn means that the don't have to get as hot to shed all the heat energy being created by the current. Thus, they don't glow as brightly as in the center.

This effect is almost certainly present at higher current levels as well; it's just that your eye wasn't able to notice the difference because it was too bright.

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The brightness of a filament is roughly proportional to its temperature. Therefore, a dimmer part of the filament has lower temperature. What's probably happening here is that the ends of the filament are losing heat by conduction to the wire past the junction.

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