Fluorescent bulb lit by coiled extension cord?

May 17, 2007 4:31 PM

Lighting a fluorescent bulb by just wrapping a plugged in extension cord around it. I remember this. Can't find an example. Help me?

I don't remember if it was plugged into anything other than the wall. Seems it would have to be. Can't find a web page, youtube video, anything. Am I imagining that I saw an example of this once?

(This is what got me thinking about it. Pretty cool.)

posted by gummo to science & nature (11 comments total)

You need a very strong E-field produced by a Tesla coil or a van de graaf generator to cause a a fluorescent tube to glow. Or in the case of your link, a transmission line with thousands of volts. Just an extension cord at 120 volts won't do it.
posted by JackFlash at 5:36 PM on May 17, 2007

That photo you linked to is a bit deceptive. The tubes are glowing, but nothing like as brightly as if they were in a fixture and turned on.
posted by Steven C. Den Beste at 5:56 PM on May 17, 2007

It doesn't take much to make a tube glow perceptibly in a dark space. If you plug in plausible numbers for tube length, pylon height, power line operating voltage and air resistivity, each of the tubes in the exhibit you linked to would be supplied with something of the order of a milliwatt of power; certainly much less than its 80 watt rated operating power. If you shone a torch on one of them, it would appear to be unlit.

You could probably devise a demo involving a cord wrapped around a tube, if you connected both ends of the cord to the tube contacts, and put that setup into a wicked strong alternating magnetic field. Effectively, you'd be turning the coiled wire into the secondary of a transformer. But I've never seen it done.
posted by flabdablet at 6:44 PM on May 17, 2007

I think you could do it, just not very brightly, with 120VAC. But you wouldn't probably want to use a conventional extension cord, because it has the current flowing in both directions inside of it. (The hot and return wire)

What you'd probably want to do, is get some 16AWG lamp wire, "unzip" it so you just had the single conductor (or just get some single-conductor wire made for conduits, although this may be more expensive) and make a coil out of that. Add appropriate resistance so the whole thing doesn't explode, and you'd probably have a pretty strong B field inside the coil for you to play around with.

And hey, if you couldn't get a fluorescent light to work, it would probably work well to shoot metal rings with. (Put an iron core in the coil, then drop a metal ring -- a piece of copper pipe works well -- around the outside of the coil, then energize it. Fun demo for students.)
posted by Kadin2048 at 6:52 PM on May 17, 2007

What you'd probably want to do, is get some 16AWG lamp wire, "unzip" it so you just had the single conductor (or just get some single-conductor wire made for conduits, although this may be more expensive) and make a coil out of that. Add appropriate resistance so the whole thing doesn't explode, and you'd probably have a pretty strong B field inside the coil for you to play around with.

A B-field serves no purpose since there are no magnetic particles in the tube so lots of turns of wire won't help. You need a strong E-field to ionize the gas. That requires a high voltage.
posted by JackFlash at 7:17 PM on May 17, 2007

This might help. Not quite an extension cord, but a similar principle.
posted by dg at 8:13 PM on May 17, 2007

Oops, just clicked on your link - sorry. Great minds ...
posted by dg at 8:14 PM on May 17, 2007

Most of the amazing fluorescent tube demos I'm familiar with have not involved wires at all; they all, like the one you linked to, involve setting up large voltage gradients in air, and positioning the tube along such a gradient.

In fact, if you make the voltage gradient high enough, you don't even need the tubes.
posted by flabdablet at 9:48 PM on May 17, 2007

In a typical fluorescent tube the two electrodes, one at either end, are made of a special metal which emits electrons when heated (it's thermionic); these electrons from the heated metal travel down the tube when there is a potential difference between the electrodes, striking and ionizing atoms of mercury, which produces a kind of cascade of electrons (and mercury ions moving very sluggishly in the opposite direction) which carry a significant current up and down the tube in response to an alternating voltage applied at the two ends. Light, mainly ultraviolet, is produced when a mercury ion and an electron recombine. This UV light strkes phosphors in the wall of the tube which fluoresce in visible wavelengths.

A coil of wire carrying 120 VAC wrapped around a cold fluorescent tube would do nothing to excite electron emissions from the electrodes at the ends. The magnetic field it would induce inside the tube would have some effect on any ions present in the gas of the tube-- they would tend to circulate around magnetic field lines-- but these would be very few in number at room temperature, and I think virtually no cascading effect would take place. Our retinas are apparently single quantum efficient, and as few as five quanta can be perceived (according to R. L. Gregory), but I don't think you'd see any glow at all under ordinary ambient light conditions.

You might be able to induce a glow if you could raise the AC frequency from 120 Hz to 1.2 MHz or more, but I'm not at all sure, and that doesn't sound like what you remember seeing.

However, I believe there is a possibility you might be able to get something if you ran a wire between the two electodes at the end of the tube outside of the coil. This would, in effect, turn the whole coil on the tube arrangement into a stepdown transformer, with the coil as the primary winding, and the tube with its connected electrodes as a secondary winding of a single turn. You would want the tube to be as short as possible to maximize the voltage drop per inch for the sake of the electron cascade, and you wouldn't want too many turns, because the induced voltage around the circuit of the tube would be 120 divided by the number of turns of the coil. I also think you would have to start with a hot tube (one you had just turned off) or there wouldn't be enough free electrons at the start to get the whole thing going. But if you met all those conditions, a small current could perhaps flow through the tube from electrode to electrode and produce a visible glow.
posted by jamjam at 10:40 PM on May 17, 2007

I recently slept on a boat's bunk under a very low ceiling- and there was a fluorescent fixture just above the bed. The static discharges from my fuzzy synthetic blanket made the tube light up quite noticeably in flashes, even in a not-quite-dark room. The blanket needed to be within about 4 inches of the tube to have any visible effect- I spent quite some time rubbing the blanket on things that night.
posted by wzcx at 10:55 AM on May 18, 2007

If you have a leaky microwave oven, you can hold a small tube and check for radiation leaks. I've seen it done before and I tested ours recently by moving a small "circline" fluorescent tube around the door edges. NO LEAKS!!!!
posted by winks007 at 2:00 PM on May 18, 2007