What causes vibrating filaments in Edison bulbs?
October 27, 2014 2:51 PM   Subscribe

A bar I visited the other day had old-school Edison bulbs hanging from the ceiling, and I noticed that in several of them, the filament was vibrating. And by vibrating I mean vibrating big time — swinging back and forth a full inch or so inside the bulb multiple times a second. The bartender didn't seem to know what caused it, but seemed to think it wasn't uncommon. Any idea?

Some notes:

-it didn't seem to be a resonant frequency from the music, since it vibrated the same through multiple songs
-some bulbs the filament vibrated just a tiny bit, some it looked like it was nearly whacking the sides of the bulb
-didn't seem to be related to the length of the cord, though of the ~12 bulbs hanging in a loose group it was the ones in the center that were vibrating
-it's Bar Sue in Seattle if you're wondering
-possibly something to do with alternating current frequency off by some fractional amount?
posted by BlackLeotardFront to Science & Nature (6 answers total) 1 user marked this as a favorite
Best answer: It's caused by slight magnetic fields generated by electricity flowing through the filament. As 60-cycle alternating current's electric field changes polarity, oscillating between positive and negative, the filament can be jerked back-and-forth by these magnetic forces. cite
posted by Rash at 3:06 PM on October 27, 2014 [2 favorites]

Corrosion can cause flicker in old bulbs, but for decor, it may be by design: Here's a flicker bulb that says it works with a magnet.
posted by sageleaf at 3:07 PM on October 27, 2014

Best answer: Without going into the physics of why, if an AC bulb is in the presence of a magnetic field, the bulb filament will noticeably vibrate.

You will notice this can be quite violent in an Edison bulb, because those old-school bulbs are dim enough to see the lit filament.

The next question is what are potential sources of magnetic fields in Bar Sue. Any current running through an unshielded AC or DC power line will generate different types of magnetic fields, each that can cause vibration — you might look for a power line running next to the light bulbs. Or there could be speakers near the bulbs — the magnets in the speaker could help the filaments vibrate.
posted by a lungful of dragon at 3:08 PM on October 27, 2014

Response by poster: So, oscillating fields, huh... I thought it might be A/C related, the frequency looked about right. It was too dark to take a picture and I don't remember the layout of the bulbs, but next I'm there I will check for possible field sources. Thanks! I think we're settled here.
posted by BlackLeotardFront at 3:27 PM on October 27, 2014

Bahaha, a question about my friends bar.

The bulbs aren't near anything. They're hanging on cables very far from the ceiling or any other electrical conduit/etc and not anywhere near any speakers. Just out in the middle of a big open room.

These bulbs do this when they're on a dimmer, which they are there. Crappy dimmers put out crappy power which is why you can sometimes hear humming from the dimmer or even the bulbs(or ceiling fan motors, especially). When they crank them up to full power, or even adjust the brightness a little most or all of them stop. I've asked about it before when i was hanging out there and we've fucked around with the brightness to make it stop.

it's a combo of the filaments being unsupported really, and the dimmers. I also think some of the filaments are more twisted than others since the bulbs are hand made. and wiggle one way easier or inversely are tensioned enough that they don't wiggle.

At a certain setting on the dimmer almost EVERY bulb will wiggle. Temperature in the bar also seems to be an element, because at the end of the summer on one of the "motown mondays" nights when it was like freaking 120 in there they were ALL doing it really hard except for maybe one.

But yea, buy one of the bulbs and slap it on a dimmer. Without one, it'll just light up. With one at certain settings it'll do it massively especially once it's warmed up. The power is just really choppy.
posted by emptythought at 5:52 PM on October 27, 2014

So the way dimmers work is that they chop the ossilation of the AC current. A filament normally vibrates 120 times a second but it is a smooth and rise and fall. When a lamp is dimmed at say 50% the fillement slams to one side of it's ossilation half way through then falls to 0 then slams to the other side half way through. You don't notice it with short fillements but with long fillements you will. This pdf has pictures and a better explanation if you're interested.
posted by Uncle at 9:15 PM on October 27, 2014 [1 favorite]

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