Wikipedia:Reference desk/Archives/Science/2010 March 19

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March 19

360 degree lightmeter

Is there any device or instrument that can record the intensity of light in lux falling on itself, not just from one direction like a conventional lightmeter, but all 360 degrees around both horizontally and vertically? And which, rather than merely averaging the light, can tell you the intensity with a resolution of say a few degrees? Thanks 78.151.108.166 (talk) 01:30, 19 March 2010 (UTC)

A simple trick is sometimes used in the movie/ computer graphics business to capture the lighting on a scene in order that graphics may be produced with matching lighting and thereby seamlessly blended into the live action. The trick is to take a mirrored/chromed sphere, place it into the scene where you wish to sample the light and then photograph it with the same camera and camera settings that you're filming the live action with.

You might at first guess that this would only capture half of the incoming light - but if you think about it carefully, you'll realise that all 360 degrees both vertical and horizontal are represented by some point on the sphere. (Well - almost - this assumes that the sphere is small compared to the distances to the light sources). The problem is that the angular resolution of the measurement of light shining from the 'back' hemisphere is very poor compared to the front hemisphere of the scene. Fortunately, when you're doing this for a movie, you don't care as much about light coming from behind the object as in front...so it kinda works out. If you really care - you can photograph the sphere from several directions at once.

So that gave us a way to grab all 360 degrees in a single image - converting that into lux is a problem that I (inadequately) answered in your next question (below). SteveBaker (talk) 03:27, 19 March 2010 (UTC)

We use a Li-Cor LI-193 spherical sensor (link: http://www.licor.com/env/Products/Sensors/193UW/li193_description.jsp) for underwater photosynthetically active radiation measurements. The 360-degree coverage is pretty good and I suspect by changing the calibration constant you might be able to get somewhat accurate readings in air. It is still somewhat directional, there's no real way around that, but it seems to do a pretty good job and it may be the closest thing you'll find.

You ask about getting the intensity to within a few degrees - since you ask for degrees, do you mean getting the direction of the light to within a few degrees of the circle or getting the light intensity to within a few percent or lux? Light intensity measurements in the field (i.e. anywhere but carefully controlled conditions) are going to fluctuate quite a bit. You want an average over time to smooth out those fluctuations. Wevets (talk) 14:41, 19 March 2010 (UTC)

Thanks, but I do not think I could use the LI193 as I assume it averages the light intensity from all around. I said degrees, I meant degrees. 78.147.2.253 (talk) 16:50, 19 March 2010 (UTC)

Perhaps the OP wants a spherical viewing intensity-calibrated camera. Two digital cameras mounted back-to-back, both with Fisheye lens and fixed exposure settings could suffice. Cuddlyable3 (talk) 00:16, 20 March 2010 (UTC)

The LI-193 doesn't average light intensity from different sources, but it does measure the *sum* of light sources from different directions (i.e. a source of 1000 μmol/m² sec to the right of the sensor and a source of 500 μmol/m² sec to the left of the sensor will produce a reading of 1500 μmol/m² sec rather than an average of 750 μmol/m² sec.) Don't know if that helps you at all though. Wevets (talk) 06:23, 20 March 2010 (UTC)

Using a digital camera as a light meter

If I took a digital photo and wanted to estimate the intensity of light in lux entering the camera lens from various parts of the scene photographed, then what calculations would I need to do? Thanks 78.151.108.166 (talk) 01:44, 19 March 2010 (UTC)

You really can't do it in general. The camera adjusts the exposure of the sensor depending on the brightness of the scene so a wide range of different light levels could generate the exact same number in the resulting image. However, if you can get a hold of a "Raw image format" image from the camera, it might contain a header that describes the exposure information...or not...depending on the type of camera...but even then, you'd probably need to know a lot about the exact nature of the sensor that the camera uses. This is either a tough problem - or an impossible one - depending on the exact make and model of camera that you're using. SteveBaker (talk) 03:15, 19 March 2010 (UTC)

If you know the ISO speed, the exposure value, and the average brightness of the image, I don't see why you'd need to know so much about the sensor. Every camera set at ISO 100, f/2.8, and 1/40s shutter will give approximately the same exposure. Using that information, you may be able to make a conversion to lux. Thegreenj 04:06, 19 March 2010 (UTC)

That's still overcomplicating things. Just set the exposure settings manually, calibrate using a surface of known brightness, take a picture of the scene you want to measure, and use one of the many photometry software out there to measure the brightness of the target surface. --09:33, 19 March 2010 (UTC) —Preceding unsigned comment added by 99.237.234.104 (talk)

You'd need to be careful, though. The scale a camera uses to convert what the light levels that it detects to an image close to what humans perceive is nonlinear and does vary from camera to camera; the scale is adjustable, for example, in some Raw conversion software. Edison's formula would likely produce more consistent results unless you had more information. Thegreenj 01:36, 20 March 2010 (UTC)

Most modern digital cameras will include all the data in the EXIF section of JPG file - see the bottom of my File:Carnforth_Canal.jpg for an example, the data has even been kept, although I've reduced the size in Photoshop. Software (like Photoshop) can display the EXIF data if asked. (Note:If I had cut out a section and made a new picture from it, the EXIF data would have been lost)  Ronhjones  ^(Talk) 19:45, 19 March 2010 (UTC)

Camera light sensors detect reflected light rather than incident light, which is more closely related to Lux, which is one lumen incident per square meter. It may not be straightforward to define "incident light" from "various parts of the scene." If you want a spot meter, then use one rather than wanting "incident light" from a narrow spot. Some meters like an old Gossen Luna-Pro have a little white plastic dome which slides over the light sensor to directly measure incident light in "EV" or exposure value. Then various websites give formulas for converting that EV (for a specified ISO, typically 100) to Lux. One is Lux=2.5*2^EV at [1] for ASA 100. (ASA was the predecessor term for ISO to indicate film speed). See also Sekonic support which has a conversion table from (incident?) light measured for ISO 100 with their meter, in EV, to Lux. See also photonet. Edison (talk) 19:27, 19 March 2010 (UTC)

The idea behind the question is to measure incident light coming from many different points rather than just averaging adding them all together with an invacone. Camera light sensors will detect whatever light hits them, they have zero brain-cells and do not understand the difference between incident or reflected, which is about the conventions of positioning camera and light meter and not the physics of photons. 89.242.46.75 (talk) 23:56, 19 March 2010 (UTC)

1/2 degree spot meters have been in common use for decades, and detect the light reflected from a tiny part of the scene. They are by definition very directional, and with suitable formula juggling should be able to provide the desired information. Lumens per steradian and all than. I just wish I had taken coursework and done problem sets and labs in photometry so I were as confident in it as with some aspects of electricity. Edison (talk) 01:22, 21 March 2010 (UTC)

The "suitable formula" is what I'm interested in finding. 84.13.41.17 (talk) 13:46, 21 March 2010 (UTC)

The spot meter reading from a bright or dark portion of a scene may vary by 4 fstops or so. Each reading, for a given ISO setting such as 100, will generate an exposure value, which could be plugged into the formula above for an incident meter. But I suspect that there may be a definitional problem with this approach. Edison (talk) 19:05, 22 March 2010 (UTC)

psychology

Is there a term or terminology in psychology to describe a situation in which an individual has both an undesirable behavior (like dependency on alcohol) and a desirable goal (like marrying into wealth) where such a goal can not be fulfilled so long as the individual posses the undesirable behavior under circumstance where the individual is permanently incapable of acknowledging they posses the undesirable behavior or comprehending that it is the obstacle keeping their goal from being reached? 71.100.11.118 (talk) 01:53, 19 March 2010 (UTC)

Did you choose the 'desirable goal' randomly, or did you need something specific to that situation, because I think we have a term for that. Beach drifter (talk) 01:57, 19 March 2010 (UTC)

"Desirable goal" could be writing a best selling novel or managing of owning a successful and growing business or seeing your children become wealthy or at least succeed. "desirable goal" is intended more as a general term related to financial and general success in life than to any specific or narrower goal. 71.100.11.118 (talk) 02:37, 19 March 2010 (UTC)

Bad habits are getting in the way of goals? A blind spot to one's own bad habits? Bus stop (talk) 03:03, 19 March 2010 (UTC)

It's not a psychological term per se, but I think character flaw pretty much covers it. Vranak (talk) 03:22, 19 March 2010 (UTC)

A person's character refers to the thing they would normally do. For instance, a person might be described as the type of person who never tells a lie. A character flaw then is indicated by something that a person normally does which might break some social rule such as cursing in public or entering someone's house without knocking. The flaw in their character is that as a rule they are not able to see the need to comply with social expectation or norm. 71.100.11.118 (talk) 05:12, 19 March 2010 (UTC)

I think the term you are looking for is denial. Looie496 (talk) 02:56, 20 March 2010 (UTC)

I'm quite curious how these tricks are done:

http://www.youtube.com/watch?v=hwVy_2eOfsE I've studied it a bit and just can't work it out, which is of course what makes a magic trick great but I am have always been one who wants to look behind the curtain.--Fuhghettaboutit (talk) 03:48, 19 March 2010 (UTC)

The first few tricks seem to be based on trick coins (there are more coins than there appear, but they can stick together to make one or more disappear when needed). Watch in slow motion (Youtube doesn't make freeze-framing very user-friendly, but give it a shot). You'll see that, for example at 3:15, the magician has a fourth coin in his hand before he even slams the table from the bottom (screengrab!); and after the slam, the coin-flip on top causes two coins to stick together. (They're probably magnetized, so that if shaken up or thrown, the coins will automatically go for each other). In the 1080p high quality video, if you freeze-frame on the stuck-together coin, you can actually see both coin-rims, not 100% perfectly aligned - the magician should know better than to try this trick in HD video!

When you notice this, you can start looking back at the other tricks (at 0:29, for example, the magician flips the coin in the accomplice's hand, but never actually grabs it); three coins "become" two with the stick-together-trick; and we don't really know what he's holding underneath the velvet (hint: it's the fourth coin). You can see the same sort of trick applied repeatedly through the rest of the video.

It seems that the final trick is accomplished with a trick table; there is a hole in the plexiglass off the the right (probably normally hidden under the teapot, which is on an upper, non-moving surface), and the table surface can be rotated in so that the hole slides into the work area. I suspect at least one of the audience members is a cohort of the performer. If you look closely, 6:18, you can see the performer's hand wobbling as the table surface is rapidly moved (the camera angle is intentionally obscuring this with a "face-on" shot instead of an overhead shot). As with all sleight of hand, misdirection is the magician's friend - he plays all kinds of games wiping the table and wiggling his hand, while the actual work is being done by an accomplice (who is rotating the trick surface for him). Nimur (talk) 04:52, 19 March 2010 (UTC)

Nice analysis. One clue is that there seem to be extraneous people around the table with no apparent purpose. Their real purpose must be to help rotate it into position. Also, anything on video could always use stop-action, where they stop filming, bring out a table with a hole in it, then resume. But, in this case, this doesn't seem to be necessary. StuRat (talk) 05:27, 19 March 2010 (UTC)

Learning Fourier Transform in 2 days

I learned it in advanced biochem protein course. I have a test on this next Monday. I'm really scared I don't understand anything. The instructor assigned readings but I don't understand them at all. I feel those readings assume certain background that I lack (Eg. the way that NMR is talked about in those papers is not like the NMR that I know.) I really need to understand this. Any great resources that don't assume advanced background in math, spectroscopy, etc. would be appreciated. —Preceding unsigned comment added by 70.68.120.162 (talk) 04:35, 19 March 2010 (UTC)

Sorry to start with the obvious resource, but have you read our Fourier transform article? Comet Tuttle (talk) 05:25, 19 March 2010 (UTC)

Before we can help you, it's important for us to know: do you need to know how to take a Fourier transform, or do you just need to understand the conceptual ideas behind it? Unfortunately, if you need to compute the Fourier transform of a signal, you need to be fairly proficient at integral calculus, and 2 days is a bit short to ramp up on that. (Alternatively, we can recommend some software which can do this for you - but that's probably not much help on a written test). If you just need conceptual help, you might want to start with the frequency spectrum article, which is a little more user-friendly than the Fourier transform article. Just keep in mind that a Fourier transform is simply the mathematical technique used to convert back and forth from time-domain to frequency domain. Nimur (talk) 05:28, 19 March 2010 (UTC)

This explanatory video takes less than 8 minutes. Cuddlyable3 (talk) 23:45, 19 March 2010 (UTC)

And here's EE261 "The Fourier Transform and its Applications", a ten-week/30-lecture course from Stanford, officially available online for free as part of an OpenCourseWare system. (Supposedly they charge something like $15,000 if you want to sit in on this class through SCPD - so grab it while it's free and free! Nimur (talk) 10:53, 20 March 2010 (UTC)

GREAT link to high-quality content - thanks for that, Nimur! -- Scray (talk) 16:03, 20 March 2010 (UTC)

Fourier transform 2

Like the poster above, I'd also like to learn Fourier transforms within a few days. To answer Nimur's suggestions to the poster above:

I'd like to know both the concepts behind it and how to actually perform it. I'm pretty good at integral calculus, so that's not a problem. Also, the Fourier transform article is very hard to understand by non-mathematicians and has no explanation of the principles behind Fourier transforms and why such transforms are so useful. --99.237.234.104 (talk) 08:46, 19 March 2010 (UTC)

The set of all functions is a vector space (mostly because you can multiply functions with numbers and because you can add two functions). See http://en.wikipedia.org/wiki/Vector_space . Vectors (and functions) can be expressed as coordinates relative to a certain basis ( http://en.wikipedia.org/wiki/Basis_%28linear_algebra%29 ) . This basis is not unique : you can express vectors (and functions) relative to many different basisses (that's not the correct plural of basis, is it? ). In particular, you can choose a "position basis" : $f(x)=\sum _{i}a_{i}R_{i}(x)$ where $a_{i}$ is the value at $x=i$ and $R_{i}(x)$ is a rectangular function centered at $x=i$ (this works for continuous functions too, only using integrals and Dirac functions rather than sums and rectangular functions). $a_{i}$ are the "coordinates", $R_{i}(x)$ is the "basis". Unsurprisingly, you can use another basis, the so-called fourier basis, consisting of sines and cosines of different frequencies (or complex exponentials, if you like that better). The formulas given in http://en.wikipedia.org/wiki/Fourier_transform#Definition are projection operators ( http://en.wikipedia.org/wiki/Projection_%28linear_algebra%29 ) from the position basis to the fourier basis (and vice versa) . They give you coordinates in frequency space, so to speak. Knowing the function in the position basis allowed you to see instantly what the value of the function was at a given point. Knowing the function in the fourier basis allows you to see how much of a certain frequency the function contains. It allows you to find superimposed periodicities in the function. You could for example fourier transform a calendar and you would find the periodicity for months, for years and even for leap years.

Now to actually calculate the fourier transform, first know what you are transforming: a continuous function R->R ? Use http://en.wikipedia.org/wiki/Fourier_transform#Definition . a function on a continuous interval [a,b]->R ? See http://en.wikipedia.org/wiki/Fourier_series#Fourier.27s_formula_for_2.CF.80-periodic_functions_using_sines_and_cosines (and read the rest of that article). A function on a discrete interval? See http://en.wikipedia.org/wiki/Discrete_fourier_transform#Definition 83.134.168.74 (talk) 09:37, 19 March 2010 (UTC)

Basis plural - Bases but pronounced "basees". Graeme Bartlett (talk) 22:01, 19 March 2010 (UTC)

CFL Afterglow

I have several compact fluorescent lamps at home. I noticed that they continue to glow for a few minutes after I turn them off. Big helix types glow the best. Since I cut the power completely, where does the energy come from? They get warm after working, is it the heat? I thought about heating the lamps with a hair dryer to see if they glow or not, then gave up fearing they'll break and spill mercury. 88.242.232.209 (talk) 10:17, 19 March 2010 (UTC)

A Fluorescent lamp "is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light." Both of these continue to some extent but I believe the phosphors in particularly usually continue to flouresce for a while (by both I meant the the mercury vapour remains at an excited state for a short time after you switch off the electricity). As you may guess heating the lamp won't do much useful Nil Einne (talk) 10:37, 19 March 2010 (UTC)

You'll notice that the phosphors in your (CRT-based) TV and computer monitor do the same thing. --Sean 13:39, 19 March 2010 (UTC)

It helps, though it's not necessary, to see this if all the other lights in the room are off when you turn the TV off. Dismas|^(talk) 11:33, 20 March 2010 (UTC)

Reusable chemical, cyronic or organic movie camera?

Since my study of the issue seems to show that even next to the best UHDTV digital movie cameras, chemical film still has the edge in terms of dynamic range, would it be possible to build a system that continuously recycled and reused all the "consumables" in film in real time, transferring it to digital format in-between? Alternatively, could a living eye (either tissue engineered or from a donor animal) be supported by a compact artificial life support system with the optic nerve wired in to a digital processor (with nerve growth factors on the chip) and if so, what would the dynamic range and other specifications be in comparison to high end digital movie cameras such as Red or conventional 35 mm movie cameras? Also, would a digital movie camera with its sensor chilled to cyronic temperatures have superior characteristics?80.1.88.11 (talk) 10:21, 19 March 2010 (UTC)[Trevor Loughlin]

It would just be a whole lot easier to build a digital camera with higher dynamic range. That's a relatively simple thing to do - even if there were limitations in the sensors (which I don't think is the case) - you could have (say) three or four sensors - each with different aperture and exposure and capture more dynamic range that way. However, astronomers long ago switched from film to digital imaging arrays precisely because the digital stuff is better - so I'm fairly sure this isn't a limitation in principle. We don't do it normally because it's expensive and TV displays have an even more limited dynamic range than digital movie cameras - so there is not much point. The idea of transferring film to digital "in real time" simply pushes the problem back one step to making you ask how you're going to go from film to digital without losing dynamic range. Biological eyes don't work like cameras - they don't deliver a simple grid of pixel values - instead there is a processing layer at the back of the eye that converts the image into data like what the orientation of edges are in the image and what direction they are moving in. Turning that back into a video signal would be an incredibly difficult process. SteveBaker (talk) 13:47, 19 March 2010 (UTC)

Some potential problems with using multiple digital cameras like that:

1) Having a longer exposure would tend to blur moving images.

2) You could potentially damage the most sensitive camera if pointed at a bright object.

For these reasons, I believe those using digital cameras to film TV shows instead concentrate on better lighting, so there aren't any dark corners that don't record on digital media. However, one variation on the multiple digital camera approach might be to use a normal camera and an infrared camera. The infrared would work to fill in the dark spots, since there isn't nearly as much variation in IR as visible light (unless you happen to be filming a fire). Something like green screen replacement could then be used to put in the IR image wherever the visible image is black. Note that this method wouldn't provide color for the dark areas, but I'd think a black and white area would be better than nothing, and it also simulates how our eyes see black and white in the dark. StuRat (talk) 14:23, 19 March 2010 (UTC)

In most cases the use for higher dynamic range is you have more room for "adjustment" before you output it to it's final form. This is done either in the dark-room or with high bit-depth digital files in photoshop. So for most uses converting to a lower dynamic range format on-the-fly would defeat most of the purpose.

If your on-the-fly film scanner is also operating in high dynamic range, then you've now go an HDR imager that takes one image per frame, why not just put a lens on it and use it directly?

Not that a perpetual film camera wouldn't be a cool hack, of course. APL (talk) 15:25, 19 March 2010 (UTC)

Hi 80.1.88.11

Improvements in sensor technology is I think the way to go. UHDTV will need to be viewed via a projector to appreciate the dynamic range that it can handle. As current projectors have a DR of 2000:1 UHDTV has 10 bits per channel and four sensors so that gives (I think) about four billion colours but it is limited in DR by the sensor signal over thermal noise and so is about 1000:1 or roughly half the projector performance, (not that this wont be amazing in itself!). Is this similar to how you see UHDTV? If so, back to your question. Cooling might help (as in space applications) but there is the problem of condensation to over come. However, solid-state heat cooling chips should make this as simple as in current military thermal IR scopes. And there is always the possibility that better sensor materials may get developed allowing for larger signal to noise ratios. Increasing the colour depth to a higher figure might also be possible but that might need a more efficient compression algorithm to get it all into the same transmission bandwidth. Leave the bionic eyes to Hannibal Chew (James Hong) of the Tyrell Corporation.--Aspro (talk) 17:20, 19 March 2010 (UTC)

Remembered ! The things that are (or were, maybe still is, perhaps, etc) secret in military thermal IR scopes are “Peltier elements” This section has a short bit about the CCD application.Thermoelectric_cooling#Uses--Aspro (talk) 21:52, 19 March 2010 (UTC)

PROJECT WORK

ARTICLE ON ELECTRONIC VOTING SYSTEM —Preceding unsigned comment added by ETE0362 (talk • contribs) 14:37, 19 March 2010 (UTC)

Electronic voting machine? DMacks (talk) 14:43, 19 March 2010 (UTC)

Electronic voting? Regards, --—Cyclonenim | Chat  14:44, 19 March 2010 (UTC)

ignitor and voltage

I was trying to make an ignitor, but the problem arose was that i used 18 volts(two 9 volts battery in series) still there was not even a speck of spark. I cant figure out why? I even tried thick aluminum wires and also tried simple copper wires. will spark produce at a particular voltage?

one more question - spark of what voltage can be easily felt by a human. i dont think this question very correct, but i have seen shock giving chewing gum packets and also shock giving tic toc pens, pens use 2 1.5 button cells ..........thanx--Myownid420 (talk) 17:15, 19 March 2010 (UTC)

You might want to try a lantern battery for a visible spark. As for what a person feels, that depends. A weak battery placed with it's contacts on the tongue can still cause a tingle. StuRat (talk) 17:42, 19 March 2010 (UTC)

Be aware that electricity can kill people. Someone with a pacemaker might be especially vulnerable, so I mention in passing (making every assumption of the questioner's good faith) it is not a good idea to build a gadget which gives unsuspecting people a shock. If you don't know much about electricity, it is very easy to make a fatal mistake, especially if experimenting with high voltages or things which can supply high current. A lantern battery produces 6 volts, the same as 4 AA batteries in series, but can produce much more current for a longer time. For information about a commonly used low current, high voltage spark source/ignitor see Piezoelectricity#High voltage and power sources, about crystals which produce a brief high voltage when they are mechanically struck. A circuit with inductance in it produces a spark at the opening of the circuit, as the inductor resists the decrease in current by generating a counter-EMF, so you might be interested in Inductor. A Transformer can produce a high voltage as well. A magnet moving rapidly near a coil can produce a high voltage, and is the basis of some toy shock trick devices, as well as the magneto on gasoline engines which produces the spark in the spark plug. It is hard to accurately define the minimum "spark" that can be perceived, since a tiny spark could produce enough heat to be felt. You could check Google Book Search for experiments where the minimum perceptible voltage for feeling a shock was measured, but it will depend on the surface area and the resistance of the skin at the site. Under the same conditions of walking across a wool rug in winter (which builds up a static charge on the person) and touching a doorknob (which discharges the electricity), I have noticed that touching the knob with the extended finger produces a painful shock, but touching it with a key held by the fingers produces no discernible shock, because of the large surface area and low resistance through which the electricity passes from the key to the hand. The threshold for perceiving electricity is usually measured in term of current, and is under one milliampere (varying for frequency). I will not state a "safe level" of current, since individuals vary. I expect that if a shock gadget operates on two 1.5 volt batteries, there is circuitry to step up the voltage many times. Edison (talk) 18:52, 19 March 2010 (UTC)

To give a sense of perspective, a spark plug uses around 20 000 V. The dielectric strength of air is 310 V/mm, which means you need 310 volts per mm of separation to produce a spark. --99.237.234.104 (talk) 19:22, 19 March 2010 (UTC)

One method is to put a large value inductor, may be 1 Henry, with a low resistance inline with your sparker and battery. Yo need a battery that can supply the highest current, and then break (open) the switch, you will get a high voltage and a spark. Graeme Bartlett (talk) 21:53, 19 March 2010 (UTC)

cant a spark can be produced using just batteries and wire? and i didane clearly understood inductors -- it says 'Typically an inductor is a conducting wire shaped as a coil, the loops helping to create a strong magnetic field inside the coil' its the loops we could see in the picture or it is inside the thing it is wound up? Has the core to be there to let it work..thnx--Myownid420 (talk) 07:18, 20 March 2010 (UTC)

Batteries can produce a spark, but typically need to have a voltage of a hundred volts or so - so you would need 10 or more of your nine volt batteries to produce much of a spark. The length of a spark is a function of the voltage - the higher the voltage, the longer the spark. Please do be very careful - once you get a voltage high enough to create sparks, it's possibly high enough to electrocute you and possbily kill you.--Phil Holmes (talk) 13:15, 20 March 2010 (UTC)

You can get a spark from a 12 volt battery or a 6 volt battery, just not a sustained spark across much of a distance, which we call an "arc." A coil of wire generally needs a core of iron or ferromagnetic material to have enough inductance to make much of a spark. A Model T ignition coil could make an impressive spark when DC current was started and stopped in the primary winging ( I expect it was in fact a transformer, like the Induction coils used in radio for "spark transmissions" before vacuum tubes were introduced. Here is the full text of a 1908 book by radio pioneer A. Frederick Collins telling all about induction coils and how to make them. (A great deal of care and skill is needed). Tesla coils also make impressive and dangerous sparks, and take a fair amount of money and skill to make. Van de Graaf Generators are available from science supply companies or EBAY and make impressive sparks. Remember that electric shocks can be painful and dangerous. Edison (talk) 01:15, 21 March 2010 (UTC)

NMR

I previously learned NMR in organic chemistry 1 & 2 and I'm taking an advanced biochem course that covers NMR. Strangely, the NMR covered in this biochem course seems like a different NMR from the one I know. Here's what I understand about 1Hydrogen NMR:

1. Expose sample to an external magnetic field Bo, which sets up the energy difference between higher and lower energy states of two different orientations 1/2 and -1/2 for 1Hygrogen isotopes in the sample. 2. In the presence of this Bo, treat the sample with photons of different wavelengths in radiowave region. The specific wavelength which corresponds to the energy difference between the two orientations is the resonance wavelength. 3. These photons with the resonance wavelength will be absorbed by and excite whichever oritentation with the lower energy (1/2 for 1Hydrogen), and this will put those lower energy orientation isotopes to higher energy level (-1/2). 4. Now I'm unsure about exactly what happens after this point. Do the excited states spontaneously fall back to the ground state orientation, emitting resonance wavelength which we can detect and use to measure the energy difference?

Also, in the biochem course, it's said that a magnetic dipole precesses around a Bo, and then absorption of resonance frequency makes the magnetic dipole to lie on x-y plane while still precessing about z-axis. Then as time goes, this magnetic dipole is restored to its previous position (precessing about z-axis and not lying on x-y plane), and the amount of time it takes for that to happen is called free induction decay (FID). I totally don't get this. How does this precessing magnetic dipole relate to the emission resonance wavelength we see? Why does FID happen? —Preceding unsigned comment added by 142.58.129.94 (talk) 17:32, 19 March 2010 (UTC)

The idea you originally described, where you gradually scan across a range of energies looking for resonant frequencies, is the way a continuous-wave instrument works. The idea of a single pulse that polarizes everything and then monitoring its decay/precession is a Fourier-transform instrument. By mathematically processing the precession, one can determine the resonant frequencies. Our NMR article talks about these different ideas.DMacks (talk) 17:44, 19 March 2010 (UTC)

Extractor fan overrun timer

Resolved

Often in a bathroom, there will be an extractor fan which is triggered when turning on the light or (power to the) shower and which continues to run for a configurable period after the switch has been turned off. Typically, how does such an overrun timer work? (I mean at one extreme it could be clockwork and at the other extreme might involve configurable software but I imagine it's somewhere in between.)--Frumpo (talk) 18:57, 19 March 2010 (UTC)

It's a circuit similar to this (fig 2):[2]. However, instead of S1 you have a current sensor switch to let the timer know that the light has been switched on. RV 1 adjusts the delay period. All these devices are all bound to be based on the very cheap 555 timer IC--Aspro (talk) 19:37, 19 March 2010 (UTC)

re-reading this I don't think I have fully answered your question so in short: The adjustment control ( inside the fan unit) is just a variable resistor! By tweaking this, one can change the rate that a capacitor discharges through the 555 timer. At a certain point, a comparator circuit within the 555 tells a flip-flop circuit (which 'flipped' on when the light came on), to now flop off. This time to discharge to the 'flop off' state can be stretched out to several minutes by adjusting the resistance (or capacitance).And is is it in graph form with the 'out put pulse' representing the fan current.

So it is all done with transistors, resistors, and capacitors and a screw-driver. You are likly to have several 555's in your home all ready. They are very handy. --Aspro (talk) 21:01, 19 March 2010 (UTC)

Perfect. I thought it must be something along those lines but wasn't sure how you'd get a switch to toggle when a capacitor had discharged by a certain amount. It's hard to search for this stuff unless you know the already know the terminology. Thanks very much. --Frumpo (talk) 16:16, 20 March 2010 (UTC)

effects of alkaline on carbohydrates

on the moore's test why sodium hydroxide doesnt react with starch and sucrose unlike glucose and fructose? —Preceding unsigned comment added by 88.242.239.194 (talk) 22:09, 19 March 2010 (UTC)

This sounds suspiciously like a homework question, and it should be noted that we have a policy of "please do your own homework". (We're more than happy to assist, but you need to demonstrate that you've at least attempted to answer it yourself, and point out what, specifically, you are confused about.) - That said, I'm not familiar with Moore's test, although I imagine your answer may have to do with the most noticeable difference between glucose/fructose and starch/sucrose (hint). -- 174.21.243.94 (talk) 21:06, 21 March 2010 (UTC)

Anti-plasmodial

I'm trying to add a link to Piper sarmentosum to the word "anti-plasmodial" (in the "In medicine") section, but I'm having a hard time figuring out what "plasmodial" means in this context. Any ideas? Thanks. howcheng {chat} 22:13, 19 March 2010 (UTC)

An anti-plasmodial is an agent which acts against parasites of genus Plasmodium — in other words, they're antimalarials. TenOfAllTrades(talk) 22:43, 19 March 2010 (UTC)

Excellent. Thanks. howcheng {chat} 00:01, 20 March 2010 (UTC)