Wikipedia:Reference desk/Archives/Science/2010 November 29

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November 29

How well does heterodyne frequency downshifting preserve phase?

When an optical heterodyne is used to shift a terahertz signal to a lower frequency, how accurately is the original phase information preserved in a form which would be useful for interferometry? I'm particularly interested in 9-10 micron infrared; presumably that would use a CO₂ laser heterodyne -- but to which frequencies makes that range most useful for accurate interferometry, and how accurate does it turn out to be, in quantitative terms? I've read the Heterodyne and Laser Doppler vibrometer articles, [1], section III on p. 1253-4 of [2] and [3] and can't determine how well this works. Ginger Conspiracy (talk) 04:19, 29 November 2010 (UTC)

The phase will be critically dependent on the phase stability of the heterodyne. This can be measured nu the phase noise, measuring the spectrum of the supposedly stable oscillator. The idea will be to get a frequency offset you are interested in and make sure that the level of spurious signal is sufficiently below that of the carrier. However if you use the same oscillator signal to mix down with two or more points of a coherent signal, and then take the difference you will cancel out most of the noise introduced by an unstable oscillator. A 10 micron infrared is well beyond what I would term a terahertz signal. The quality that you would need is a narrowband enough heterodyne that when its spectrum is superimposed (convolved) on the desired downconverted signal you get no undesirable effects. Graeme Bartlett (talk) 08:08, 29 November 2010 (UTC)

Thanks very much. Phase noise is exactly what I was looking for. It's always a great pleasure to see such seemingly esoteric articles with such great content and external links. Ginger Conspiracy (talk) 03:35, 30 November 2010 (UTC)

Modified corn starch

When the ingredients listed for a food product include "modified corn starch", does that mean starch that was taken from corn and then modified, or starch that came from modified corn? NeonMerlin 06:00, 29 November 2010 (UTC)

The former. The starch is modified. The corn may (or may not) be modified, but that is irrespective of the name. See Modified starch. --Jayron32 06:04, 29 November 2010 (UTC)

LED Xmas light string efficiency

Thinking of buying some, and have a few Qs:

1) How does their efficiency compare with the following:

a) Incandescent Xmas light strings.

b) Regular incandescent lights.

c) Regular LED lights.

d) Regular CFL lights.

Efficiency being defined, of course, as lumens of light produced per watt of power used, but feel free to use other units, if you prefer. What I'm getting at with efficiency is knowing whether my electric bill will go up or down if I largely light the house with those strings instead of with regular lamps and overhead lights.

2) Also, how long should a string of LED lights last ? And when one light fails, will the rest continue ? Is there some point at which too many lights have failed, where the whole string will go out ?

3) As for safety, I assume that they use less energy and produce less heat than incandescent Xmas lights, so this should reduce the risk of fire, right ? StuRat (talk) 14:20, 29 November 2010 (UTC)

You are comparing mono-chromatic light to white light. The theoretical max for white light is 251 lm/W, but for a single color it's 683 lm/W (it varies a bit depending on the color). This is not just theoretical - it makes a huge difference in practice too. Since you need colored lights, LED is by far, no question about it, the most efficient. Incandescent, besides being inefficient, and hot, also looses a ton more efficiency by using a filter to make it colored. CFL can be made in a single color, but in practice it's not.

Re-reading what you wrote, are you trying to use them for general lighting? If so then CFL is best. See Luminous efficacy for chart. LED has a lot of vocal support, but it's not as efficient as CFL, and the color quality is worse too.

LEDs last about 20,000 hours, but they don't burn out, they just get dimmer. LEDs are probably wired in series. An LED needs about 5 volts, so you have about 24 of them in series for 120 volt service. I'm not sure of the failure mode of an LED (i.e. does it fail open or closed). But normally they don't fail anyway, unlike incandescents.

I'm not sure there is a lot of risk of fire from incandescents anyway (I mean you could, but in normal use it's not a problem) so that's not really a comparison. But less heat does mean a lower A/C bill. Ariel. (talk) 21:01, 29 November 2010 (UTC)

I'm not so sure about some of your claims.

For example the colour quality claim. For generic 5mm or similar LEDs sure. But high CRI (90+) LEDs do exist (see [4] for example). These tend to exist in warm white too although that's (IMHO) strong evidence that's a cultural thing so not really relevant. I don't know how these compare to state of the art CFLs but most people aren't using those (i.e. they seem satisfied with the lower quality) so it seems fair to point out you can get something which will probably be good enough if you really want. Of course CRI probably isn't perfect anyway [5] although that's somewhat of a field beyond me.

I don't know about the claim of LEDs not failing either. The 20k lifetimes are great in theory. But from what I've read many LEDs lights aren't living up to their purported lifes. In particular one problem is given the common high variance in Vf if these are matched into parallel arrays without proper regulation, some LEDs tend to be overdriven, these eventually fail and then the others are then overdriven so the how thing quickly fails. [6] [7] [8] I've also read of very quick and rather dramatic dimming of cheap Chinese white LEDs (mostly the 5mm etc kind) even with rather low non over-driven currents. LEDs have a big problem in they really don't like heat, it reduces the efficiency, changes the colour and drastically shortens the lifespan, so incandescent bulb replacements are still problematic. (Enclosed fixtures are usually a case of just don't get, except perhaps when it's purposely designed for LEDs.)

In terms of luminous efficacy, while CFLs probably still win here I don't think they will stay much longer, our article currently has 93 [9] which is over a year old. This compares to a best of 120 for CFLs from our article. But LEDs are advancing at a far faster rate AFAIK (compare the Cree XM-L LED launched recently with the XP-G launched last year for example [10], while these are raw LEDs the bulbs are clearly going to trend). Of course if you want high CRI or warmer whites you're worse off and I'm not sure how LEDs luminious efficacy compare with CFLs when at that level.

Price wise LEDs still have a way to go, particular since as I mentioned they may not last as long as sometimes claimed.

Nil Einne (talk) 22:05, 29 November 2010 (UTC)

What you wrote doesn't really dispute my claims. I have never understood why people keep saying LEDs have better color, CFLs and LEDs make colors by the exact same method! LED's have no advantage here. They make UV and phosphors change the color. CFLs do exactly the same thing. The difference is that CFLs generate a deeper UV, which helps a lot. I simply check the packages and displays in stores, and it's no contest - CFLs are far better. That post you linked says essentially "LEDs are not doing so well in color accuracy, so lets redefine color accuracy so LEDs do better".

On top of that color accuracy and efficiency are enemies of each other, so it doesn't help to compare a high efficiency 70cri led. Notice the Toshiba bulb: 93 lm/W is at 70 cri. If you want 90 cri it's only 69 lw/W which is much worse than a CFL for that CRI. The 160 lm/W is only at very very low power levels, and no mention is made of CRI. I don't agree that LEDs are advancing faster. They are advancing fast in total lumens, but in efficiency it's at a much slower pace (and remember low cri doesn't count). Maybe one day LEDs will be better, and I'm glad there are people funding the research by buying poorly performing LEDs, but I'm not going to (OK, actually I did buy an LED for a bedside lamp for kids, since it's unbreakable :). Ariel. (talk) 23:29, 29 November 2010 (UTC)

Long semi OT reply to IMHO questionable claims Nil Einne (talk) 18:32, 30 November 2010 (UTC)

Who said they have better colour? I didn't. Nor did StuRat. You however did claim they have worse colour, which I stick by my claim is questionable. For starters there are high CRI LEDs. Are these worse then the best CFLs? I don't know. You definitely haven't presented any real evidence that there are. Your anecdotal evidence is somewhat irrelevant since 1) People have different preferences 2) You haven't indicated how you compared, did you use colour swatches? Was any semblence of a blind test done 3) We have no idea what sort of LEDs were being compared, were high CRI LEDs even involved? Nor is it clear what CFLs are being compared. Now it may be that there aren't any real high CRI replacement bulbs, LED bulbs are still somewhat of a niche product as I've indicated. And as I've said, AFAIK the evidence suggests most people don't care that much about CRI so it's hardly surprising that of what does exist, manufacturers are concentrating on what's good enough for the majority rather then what a minority at the current time may want. That bit about people not caring that much is of course an important point. I admit I don't have clear evidence here but as I already said, most CFLs aren't that high CRI. In NZ the CRI isn't usually even advertised or specified from what I've seen. High CRI CFLs do exist but they tend to be niche products AFAIK. Now as I said, I don't have clear evidence like sales figures so I could easily be wrong, but I don't think I am. Given this, the claim that LEDs have worse colour is even more misleading since from most peoples POV you can get better CRI LEDs then what they're currently using in CFLs. This doesn't mean LEDs have better colour, I never said they do, it just means it's misleading to say CFLs have better coloru as you have done repeatedly. As for the CRI definition thing, well I said it's somewhat beyond my expertise. However I freely admit I trust actual published research, much more then I do claims by random wikipedians backuped by nothing so I'll trust the research suggesting CRI, something developed in the 30s and 60s may not be perfect. Even more so when the only apparent response said wikipedian has is that "LEDs are not doing so well in color accuracy, so lets redefine color accuracy so LEDs do better" yet some of this research was from 1995 and attempts were made to reform CRI itself in 1991-1999. Blue LEDs had started to gain interest by 1995 so I guess it's possible that people in 1995 were trying to fix the standard to give LEDs a leg up again CFLs but I admit wacky conspiracy theories have never been my thing. (This is not to say I'm denying such things tend to be very political our article itself says "The committee was dissolved in 1999, releasing (CIE 1999), but no firm recommendations, partly due to disagreements between researchers and manufacturers"). Ironically from what I've read one complaint possibly about CRI or at least luminous efficacy is that it underemphasises the blues which makes me wonder about your UV claim. I'm also a bit confused about the relevance of your anecdotal evidence anyway, surely all that matters if you think CRI is perfect is the CRI and colour temperature (and luminous efficacy), why worry about how they look? If you get equivalent CRI and colour temperature to what you prefer CFL and LEDs then these are the same and just compare the efficacy. Unless you are acknowledging that CRI isn't perfect. Note I already said you lose efficiency if you want increased CRI (or lower colour temperature) so I'm not really sure why you're repeating what I said. You've claimed a lower CRI LED is much worse then a low CRI CFL but provided no evidence. But rather then waiting for some, let me try to find something. For starters your claim is misleading anyway since the 69 lumens per watt is warm white not cool white which the 70 CRI is so it isn't just the higher CRI but the higher colour temperature. The 69 lumens per wat BTW is also 80 CRI not 90 CRI. But looking for highish CRI CFL, I was directed to (well the main site) [11] where we see EnergyMiser warm white 600 lumens 80 CRI with 55 lumens/w (ooops!). But moving on perhaps that's not totally fair since there's also a 68 lumens per watt 820 820 lumens warm white 80 CRI and a Phillips 700 lumens warm white 82 CRI 64 lumens per watt and uh lets ignore the TCP. The Toshiba doesn't specify what they mean by warm white so I'm presuming of course they mean 2700K like the others but I'm not that sure (I did find some claims the Toshiba was 2700K but none from Japan). While the CFLs here don't actually come out better I'm not suggesting these have the highest efficacy so I'll concede as I did in my first post CFLs probably still win. Particularly since, I've seen (a while before this this post) some evidence the Toshiba figures may be somewhat inflated (although I've read that's not uncommon with CFLs either) and I've also read that the Toshiba's in Japan have shit power factor and other issues because of lax standards in Japan and meeting the stricter standards in the EU and US lowers efficiency (I'm presuming the CFLs meet the stricter standards since that's a US site) [12]. (Another problem is of course heat as I've already emphasised, your LED bulb may be 600 lumens when you first turn it on, I wonder if it will still be that 8 hours later in a low airflow room and a somewhat enclosed space.) However as you may have guessed, I am awaiting support for your claim 'which is much worse than a CFL for that CRI'. You're right that low currents LEDs have greatest efficiency at low currents, this was perhaps not something I made clear enough in my original post. I'm ignoring your irrelevant claim that low CRI doesn't count since as I've already said while your welcome to your POV, there's no evidence it's held by the majority of people. The XM-L is still not really widely available so it's difficult to get good comparisons but from what I've seen 10% efficiency gain isn't unresonable. 10% may not seem like much to you but I'm sure quite a lot of industries will be happy with a 10% efficiency gain a year. I'm not aware CFLs are advancing at anything close to this rate. P.S. One of my earlier comparisons was a fluourescent light not a CFL which perhaps wasn't the fairest but since these are commercial available bulbs (as opposed to raw LEDs) I'm still willing to let it stand. I would note our luminous efficacy article suggests 46–75 for CFLs. P.P.S. The website I used above showed CFLs all had an advertised CRI of 80 or higher (although I was directed there when looking for highish CRI). My impression had been that many CFLs have a CRI of about 70-75. However I appear to be mistaken on this, for example I found [13] which shows all CRIs tested there (Australian) were 80+ and it is in fact the ANZ standard. I also find [14] which suggests ~80 is the norm. In that case, and given a CRI of ~70 seems the norm for cheap or generic LEDs I'll concede the average LED is worse then the average CFL. However it still doesn't change the fact you can get LED light bulbs better then the average CFL and in fact approaching the best CFLs e.g. [15]. (Efficacy wise that's 54 lumens/watt so not that great although I still don't know what the CRI for high 90+ CFLs are. Note that the Australian study I linked to found the luminous efficacy was around 60-80 lumens/watt for the 80.4-87.7 CRIs in that study so I'm still not seeing much evidence for these far better efficacy of CFLs that was alleged. These are real world independent tests however from my brief read thorough I think they found most manufacturers live up to their claims so I'm not sure if the LEDs are going to be much worse. Perhaps this is a matter of semantics, I don't consider 20-30% far better, particular considering the rate of advance of LEDs but perhaps you do. ) Nil Einne (talk) 18:32, 30 November 2010 (UTC) Maybe high CRI, high efficiency LEDs exist, but they are certainly not commonly available - I always check the lamp displays, and all the LEDs on display are very blue. And many of the LEDs don't even list lumens, which I find quite shady. Additionally the CFLs will say "warm/cool white", etc. The LEDs don't, they leave you to guess. I think you are arguing about LEDs that exist, and I am arguing only about those that I can find in a hardware store. Right now, if you go to buy a bulb in a store, CFLs beat LEDs, but I'm willing to accept that it's possible to buy great LEDs online. Ariel. (talk) 19:28, 30 November 2010 (UTC) Ok thanks for the clarification. I suspect however it depends where you live, I belief for StuRat this is the US so perhaps your comments are somewhat appropriate for him. But it wouldn't surprise me if the Toshiba linked above was available in some Japanese hardware stores. In NZ I don't think I've even seen a mass market retail store selling LED bulbs, nor in Malaysia (although haven't looked that well there, it's not something that interests me) so if you want LED bulbs buying from specialised stores is probably your only choice. Some of the bulbs, e.g. the Cree LR6 aren't really intended for the mass home market anyway given heat, cost and perhaps weight issues. Note in case this wasn't clear enough I wasn't trying to suggest LEDs are better at the moment, just that in terms of CRI, CCT and efficacy I don't think they are really that far from CFLs. Realisticly, the number of people interested in buying $100+ light is small (particularly given the care needed, the small number of advantages and the fact 5-10 years from now people will be laughing at the old prices) hence as I mentioned they still have a long way to go on price. Nil Einne (talk) 20:12, 30 November 2010 (UTC) (EC) Semi answering my own question I find [16] with a 13W CFL with CRI 93+, 780 lumens, 5500K CCT. That's 60 lumens/W in other words the Cree LR6 linked above doesn't seem that bad in comparison particularly noting it was launched in 2007 or 2008 from what I read (not sure whether it's undergone improvements since then). Nil Einne (talk) 20:12, 30 November 2010 (UTC)

Thanks for the answers so far. I had strings of LED Xmas lights in mind for two purposes:

A) Night lights. Should be more efficient than the incandescent night lights I use now. These are just so I can find the light switch without smashing my toe on the coffee table, etc. I do have one CFL night light, and I will probably keep that one.

B) Area lighting. I don't like having a single bright spot in the room, which either an overhead light or table lamp produces. Strings of Xmas lights would give a more even lighting.

As for the color, I'm flexible there. If I wanted white, overall, couldn't I combine different colored LED lights to get that ? StuRat (talk) 05:24, 30 November 2010 (UTC)

For evenly lighting a room, you might consider up-lighting. We have a room that was previously lit by small lights all 'round the walls, and now we have a bright free-standing lamp that directs its light in a cone facing the ceiling. It's maybe 4 or 5 feet lower than the ceiling. The light reflects off the white ceiling and evenly lights the whole room very satisfactorily. It's very impressive if you haven't tried it before. 86.161.109.130 (talk) 11:11, 30 November 2010 (UTC)

I can't offer that much advice since it's not something I've explored that much (I'm more interested in the flashlight arena). You may find some advice here [17] although be warned I've sometimes found the culture there fairly elitist and pro-American/anti-Chinese. However I've also seen a lot of fairly complex discussions about lighthing including custom solutions. (I'm not sure whether that's really what you're going for since it's likely to be rather expensive.)

My guess is using the white locations and right lens, reflectors etc will produce a far better result then a lot of small Christmas lights.

You can get RGB LED lights like this [18] but they tend to be orientated to having different colours rather then mixing the right white light for your purposes (although you may be able to do that, I don't really know). They also tend to be expensive, low powered and I expect not that efficient. I'm not particularly sure they give great CRIs either whatever you do. I suspect just having different colour lights won't work very well unless you are going for a disco/nightclub effect.

Nil Einne (talk) 18:50, 30 November 2010 (UTC)

Hmmm, "the white location" ? :-) StuRat (talk) 22:40, 30 November 2010 (UTC)

Computer modelling of Artificial consciousness

"Computer modelling shows that even consciousness can be generated with very small neural circuits....only a few thousand could be enough to generate consciousness." from http://www.sciencedaily.com/releases/2009/11/091117124009.htm

What computer modelling is this? I've tried searching. Thanks 92.24.176.72 (talk) 14:32, 29 November 2010 (UTC)

It's probably an artificial neural network. I've taken the liberty of cross-linking this to The computing ref desk. CS Miller (talk) 16:15, 29 November 2010 (UTC)

That popular article was based on this scientific paper. Discussion of artificial neural networks in that paper starts on page 7. The popular article is apparently being sloppy and exaggerative when it refers to the kind of cognitive tasks performed by artificial neural networks as discussed in the paper as being "consciousness". The paper itself does not use the word "consciousness". Red Act (talk) 16:25, 29 November 2010 (UTC)

Which is funny because "consciousness" is precisely the wrong way to describe what is going on. It's not surprisingly that you can make a small neural network count; counting is computationally easy. When humans count, they don't have a special counting neural network that they can invoke. If you tell someone to count by threes, or to stop counting and start again later, writing their number down so they don't forget it, they'll be able to seamlessly make the appropriate changes to their mental algorithm. So humans are able to reflectively modify their own cognition. Consciousness, normally defined as awareness of self, is the hard part. Executing little algorithms is easy, regardless of the substrate (neural networks, the symbol-manipulation of intelligent brains, or assembly language). Writing them is hard. A bee brain may be capable of counting, but I really doubt they can decide to count, or figure out how to count. Paul (Stansifer) 16:55, 29 November 2010 (UTC)

The problem of course is that we don't have a good operational definition to test for consciousness. This problem is discussed in our testing section of the Consciousness article. Nimur (talk) 19:00, 29 November 2010 (UTC)

Role of salt in ethanol precipitation of DNA

I don't understand the role of the salt. Does it cause the phosphate groups in the DNA to dissociate, giving up a proton? Or else if the the phosphate groups are already in the deprotonated form, why is the salt needed? Gidip (talk) 18:18, 29 November 2010 (UTC)

I was told it was because you needed a counter ion for the phosphate groups. In solution at neutral pH, the protons are already dissociated from the phosphate groups. Because of electrostatic repulsion, you can't force just the charged DNA into the solid phase - you need to counter the charge with a positive ion. The salt solution provides enough counter ions for the DNA to come out of solution as a neutral salt. (Note that DNA behaves the same as any other ionic species in this regards.) -- 140.142.20.229 (talk) 18:36, 29 November 2010 (UTC)

Well heck, we've got an ethanol precipitation article. DMacks (talk) 20:33, 29 November 2010 (UTC)

The net charge in the system has to be zero. Therefore, for every negatively charged dissociated phosphate group there is a proton. Then what's the salt for? Gidip (talk) 08:10, 30 November 2010 (UTC)

If they have a high propensity to dissociate (i.e., acidic), you would have to lower the pH to make that happen effectively. But you're right, if the solvent becomes non-dipolar enough, they would be more likely to stick. But they would stick as a covalent OH (not just ion-paired). Neutral covalent compounds are generally soluble in organic solvents, so you haven't accomplished the precipitation: the goal is to make the DNA insoluble, not just into some net neutral complex. So instead, seems like the goal is to promote formation of ionic salts of the phosphate. DMacks (talk) 09:16, 30 November 2010 (UTC)

Red bleach stains

Using laundry bleach, I occasionally splash some on my clothes, and get a stain. Typically the stain is lighter than the fabric, but I've recently encountered some odd exceptions:

1) A black pair of jeans had bright red dots where it was splashed. I guessed that the black color was, in fact, made up of multiple colored dyes, and that the red component was more bleach resistant than the others.

2) A white knit shirt, with a collar made of a different material, had the white collar turn pink (a pale rust color, to be more specific), while the rest of the shirt was unaffected. I can't use the previous explanation here. The rust color makes me think that there was actually some iron in the collar, which then oxidized when exposed to bleach. Is there any white dye which contains iron ? StuRat (talk) 20:55, 29 November 2010 (UTC)

I think your explanation for 1) is correct. As for 2), was the collar uniformly pink, or in a characteristic `splash' shape? If the former, I propose color was released from other fabrics, and preferentially bound with the collar. My understanding is that white cloth is not generally made these days by dying with white dye, but by bleaching off-white source material. SemanticMantis (talk) 23:04, 29 November 2010 (UTC)

2) It's the splash shape, any ideas ? The shirt might be very old, I inherited it. Also, the red showed up immediately when the bleach hit it. StuRat (talk) 05:07, 30 November 2010 (UTC)

Fusion and fusion

what I don't understand is were this energy comes from. Here's what I mean. The reaction of deuterium and tritium goes like this:
²H + ³H → ⁴He + ¹n + energy
Were does this energy come from? You are left with less mass than you did before, but you still have three neutrons and two protons, just like before, yet they are lighter. I don't see how you can get "light particles"; a proton is always a proton, with three quarks, right? --T H F S W (T · C · E) 21:42, 29 November 2010 (UTC)

I believe the answer is in the Binding energy. Vespine (talk) 21:57, 29 November 2010 (UTC)

Then why is there mass loss? Does the binding energy hover around the particles like dark matter around a galaxy? --T H F S W (T · C · E) 22:11, 29 November 2010 (UTC)

The loss of mass and release of energy are the same. I think this is ultimately the same reason a compressed spring has more mass than a spring at rest, i.e. E=MC^2. SemanticMantis (talk) 22:57, 29 November 2010 (UTC)

Exactly. If you need a particle to imagine being converted into energy, the mesons mediate the binding energy of the nuclear force, but they are very short lived, virtual particles, so it may make more sense to just think of conservation of energy. Ginger Conspiracy (talk) 23:14, 29 November 2010 (UTC)

I suspect the difficulty in visualizing this is because the popular image of "energy" is as a little lightning bolt, i.e., electrical energy. "Energy" is a far more diffuse and diverse concept though. Personally I think our way of visualizing nuclear reactions is harmed by our persistent representation of the energy released as a little lightning bolt, when it is really released as kinetic energy in the speed of the constituent parts. In any event, thinking of energy as a "thing" that hovers around (or zaps out) is going to mislead you every time. --Mr.98 (talk) 00:19, 30 November 2010 (UTC)

To answer the question, the energy in this case is almost certainly released as heat energy; the fact that the alpha particles are moving after being released means they have energy. The speed at which they move will be ultimately related to the amount of energy released by the reaction. Heat energy is just the energy of molecular motion. --Jayron32 04:14, 30 November 2010 (UTC)

The nuclear binding energy is actually negative, but that's not important; what's important is that the binding energy per nucleon is different for different elements (and isotopes). If you plot it as a function of nucleon count, you get a bowl-shaped curve with the lowest point at Iron-56, as shown here (upside down). Thus, you can liberate energy by putting small atoms together or by splitting big ones apart. -- BenRG (talk) 08:34, 30 November 2010 (UTC)

is this true?

http://i.imgur.com/GBGAY.gif
can you really build it like that or is it just an animation? 82.234.207.120 (talk) 21:43, 29 November 2010 (UTC)

Kentucky Do-Nothing. Plans are here. Here's the Facebook page. Buddy431 (talk) 21:49, 29 November 2010 (UTC)

Thanks! Now I finally have a name for those things. I had one when I was a kid but never knew the name. Dismas|^(talk) 07:12, 30 November 2010 (UTC)

Also known as a "Do-Nothing Machine" or "Bullshit Grinder", or more formally, as a "Trammel of Archimedes". When used to draw ellipses, they may be known as an ellipsograph. Buddy431 (talk) 20:11, 30 November 2010 (UTC)

I see that when the angle of the rod changes uniformly, there is simple harmonic motion in both axes. Is there a simple way to regulate the motion of the device so that it traces out an elliptic orbit around one focus according to Kepler's first law? (Despite some material at orbit equation, eccentric anomaly, true anomaly, etc., I should say that deriving this from scratch doesn't look easy...) Wnt (talk) 11:16, 30 November 2010 (UTC)

What exactly do you want it to do? You can easily change the size and shape of the ellipse it produces. See this demo. Buddy431 (talk) 20:14, 30 November 2010 (UTC)

Too many birds on a power line

Yesterday, I looked outside and saw that the power line to our house was completely filled with ravens. This wire is about 100' long, and was sagging quite a bit from the weight of the birds. I've lived here 17 years, and have never seen that many birds on one power line before. I have two questions: 1) what would cause that many ravens to congregate on my power line, and 2) is it possible for the birds to actually break the wire? Shuttlebug (talk) 21:44, 29 November 2010 (UTC)

The wires are strong enough for a person to hang from them, so I doubt birds could hurt it. Ravens like to group. I've frequently noticed large flocks right before storms. Ariel. (talk) 22:06, 29 November 2010 (UTC)

Probably planning a murder. HalfShadow 22:07, 29 November 2010 (UTC)

groan. DMacks (talk) 22:10, 29 November 2010 (UTC)

(edit conflict)Question1: I suspect that it was either 1) a group of young birds or 2) a group of males looking for females.

Question2: No. Metals, such as those cables are made of can be stretched and bent a long way before snapping. I have seen trees fallen on power lines and the lines have been almost touching the ground (of course, the poles were bent too). Are you sure that the birds were making it sag? Power lines always droop a bit. --T H F S W (T · C · E) 22:09, 29 November 2010 (UTC)

To answer question 2) Engineers who design these things are not complete idiots. Birds are known to congregate on power lines, and I am certain that the strength of the line is designed to take this into account. --Jayron32 04:11, 30 November 2010 (UTC)

I believe there was a documentary produced about ten years ago that studied this problem in depth. Wikipedia has an article about it if you're intererested. —Bkell (talk) 12:52, 30 November 2010 (UTC)

Ha, ha, Bkell, very nice connection.

Seriously, overhead power lines are constructed to some pretty amazing engineering standards. I don't know if bird loads are ever specifically accounted for, but in areas where it freezes, the standard is that the line must be able to support the weight of an ice coating up to five times the wire's diameter. In areas subject to extreme ice storms -- such as Quebec -- the standard is raised to ten times. (And yet, they are still occasionally pulled down due to stupendous quantities of encased ice. [19])

One of the most amazing demonstrations of the "hoisting capacity" of an overhead power line occurred in Seattle in April, 1998, when a small private airplane got tangled in one and dangled from it for a couple of hours [20] [21] before some very intrepid firefighters effected a one-of-a-kind rescue. [22] —Steve Summit (talk) 13:20, 30 November 2010 (UTC)

I think I need a small vacation/But it don't look like rain/And if it snows that stretch down south/will never stand the strain. --Trovatore (talk) 03:49, 1 December 2010 (UTC)

Not all power lines are the same. The one which supported the plane was 50 feet off the ground and supplied a steel mill and 2500 customers. It was likely a transmission or subtransmission high voltage line, and had little in common with the line carrying power to ones house from the transformer, or in many cases carrying power to the local transformer. The high lines may be steel reinforced, while the line in the alley or along the highway in front of your house may be far weaker. A 4kv or 12kv feeder might be relatively small copper wire. The service going from the transformer to a house might be rather small as well. Even so, it is hard to envision bird loading sufficient to break wire that stands up to wind and ice. Edison (talk) 17:20, 30 November 2010 (UTC)

Fluorescent light flashing after being switched off

The energy-saving fluorescent light in my kitchen has just behaved rather strangely. Just now I switched it off, after which it flashed at intervals of approximately 1 second, for a total time of about a minute. Right at the end of this just one of the three "loops" was flashing. What could cause this? Thanks, DuncanHill (talk) 22:15, 29 November 2010 (UTC)

The modern Compact fluorescent lamp have voltage control circuitry (the old ones chokes) so that they can run between 230 -ish volts and about 256 -ish volts. Once the power is off, it might just be the capacitors discharging. The reserved charge is dissipated each time the arc is achieved. After a few flickers the capacitors are completely discharges. Also, maybe you lighting loops are wired in the 'live' configuration. Nothing wrong with this (according to the UK regs) but important that you make sure the power's off before allowing you pinkies any where near the copper.--Aspro (talk) 22:42, 29 November 2010 (UTC)

Oh! And is your kitchen very cold. Dry air and low temperatures allow florescent tubes to exhibit behaviours not seen at higher temperatures. --Aspro (talk) 22:45, 29 November 2010 (UTC)

Has been very cold lately, and I think there's cold air coming into the flat above. DuncanHill (talk) 23:09, 29 November 2010 (UTC)

I asked this question just the other day! Zunaid 13:36, 30 November 2010 (UTC)

Antonov AN225- Personal experience and photographs available

Hi there,I flew on the AN 225 from Manchester in 2006, and have a little knowledge and plenty of photographs of it should they be of interest? —Preceding unsigned comment added by Filler9 (talk • contribs) 23:13, 29 November 2010 (UTC)

Please consider licensing your images and uploading them to Wikimedia Commons. Read Wikipedia:Uploading images and be sure you understand the implications of licensing your images; then use the uploading form to put them up. Also check out our article, and feel free to edit it to add content; but be sure you understand our verifiability policy and do not contribute original research. Nimur (talk) 23:21, 29 November 2010 (UTC)

(ec) Please consider improving the Antonov An-225 article and/or uploading your photos to the commons:Antonov An-225 Wikimedia Commons category. Ginger Conspiracy (talk) 23:22, 29 November 2010 (UTC)