Science desk
< September 1	<< Aug \| September \| Oct >>	September 3 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 2[edit]

Photoelectric effect[edit]

in photo electric effect we prefer to use alkali metal .why? instead of alkali metal can we use liquid ? — Preceding unsigned comment added by 49.126.0.33 (talk) 02:49, 2 September 2015 (UTC)[reply]

I don't understand the question. Wikipedia has articles about the photoelectric effect, alkali metals, and liquids. I'm not aware of any connection between those three concepts in the way you are asking about. --Jayron 32 17:43, 2 September 2015 (UTC)[reply]

Alkali metals have the lowest work function and by this reason they are used in cathodes including photocathodes. As to liquid photocathodes, they are mentioned in the literature. Ruslik_Zero 17:53, 2 September 2015 (UTC)[reply]

I thought as much, since alkali metals also have the lowest ionization energy, which means they have little resistance to liberating electrons. --Jayron 32 18:08, 2 September 2015 (UTC)[reply]

Also, liquid metal is still metal. Thinking of metals as "solid" is anthropocentrism at work. Not to mention, mercury, gallium, some of their alloys, and NaK are liquid around room temperature. Now, I'm not sure how heating metals affects their photoelectric properties; my point is that there's no fundamental change in the substance. --71.119.131.184 (talk) 21:12, 2 September 2015 (UTC)[reply]

I should add that the benefit of the low ionization energy is that the photoelectric effect can be studied in alkali metals using visible light, whereas ultraviolet would be required for many other materials. I would speculate that it would have been harder to control which wavelength of UV using a prism, since many types of glass absorb it, and of course it isn't as easy to look and see what is happening with the light. Wnt (talk) 11:45, 7 September 2015 (UTC)[reply]

Screen energy demand on laptop[edit]

Does it make a difference in energy consumption for my laptop to choose a more dark than light screen image or the reverse, a more light than dark image? I am trying to extend battery use, and since the screen image is rather permanent in the background it might make a difference, or not? --VanBuren (talk) 13:11, 2 September 2015 (UTC)[reply]

It depends on what type of screen you are using. With an LED screen, the backlight is always on. With AMOLED or OLED screens, black will use less power. Your laptop almost certainly uses LED, and the best you can do to save power is to turn the brightness down. 217.158.236.14 (talk) 13:17, 2 September 2015 (UTC)[reply]

Thank you! --VanBuren (talk) 19:42, 2 September 2015 (UTC)[reply]

Other things to save power: close the lid whenever you're not using it, and ensure your laptop is configured to enter sleep mode or hibernation when you do so. And ensure wireless is disabled when you're not using it, if possible. Many laptops have a physical "kill switch" for the wireless. Unplug any peripherals, like USB devices, when not using them, because they draw power. Also, no big deal, but we do have a computing reference desk for computer-specific questions. --71.119.131.184 (talk) 20:51, 2 September 2015 (UTC)[reply]

I think you mean an LCD with LED backlighting. (OLED screens are by definition LED screens. Calling LCD screens with LED backlighting LED screens, is IMO more marketing than truth.) And while the backlighting may be always on, that's only half the story. It's definitely possible for the backlighting to be dynamicly locally dimmed, although it primarily happens to try and improve the dynamic contrast, mostly only happens in large TVs (AFAIK) and the number of regions is often generally small (perhaps 4); and so the power savings is likely to be small. See LED-backlit LCD display and [1] for some discussion. Nil Einne (talk) 12:43, 3 September 2015 (UTC)[reply]

Yes, I misspoke. I did mean LCD with LED backlighting. I should post less when I'm half asleep! 217.158.236.14 (talk) 16:10, 3 September 2015 (UTC)[reply]

I think the responders may have misinterpreted the question. I thnk the OP is talking about the screen's wallpaper image and asking if a light or dark wallpaper will make a difference to the power consumption - Arpingstone (talk) 07:10, 7 September 2015 (UTC)[reply]

No that seemed to be what we were all talking about. Nil Einne (talk) 20:08, 7 September 2015 (UTC)[reply]

Which paper at sciencedirect for this Lancet paper?[edit]

I'm looking for the article "The case of scirrhous of the prostate gland with corresponding affliction of the lymphatic glands in the lumbar region and in the pelvis" in the Lancet from 1853 by Adams. Page 393, so I believe it's either doi:10.1016/S0140-6736(02)68759-8 or doi:10.1016/S0140-6736(02)53862-9 but I cannot see the full text or first page to confirm, and cannot find this Lancet issue in archive.org. Would somebody with full access to sciencedirect or thelancet.com please confirm which it is? Thanks Rjwilmsi 16:45, 2 September 2015 (UTC)[reply]

It's the first one. To clarify - [2] goes to here [3] and then I get this pdf [4] which is paywalled, but has the title you quote (if you don't have access I think that redirects to a prior link). The other is something about a report on cholera. You can ask at WP:REX for the pdf, or you could contact me via email on my talk page. I don't know what's going on with the confusing DOI. But hey, I just skimmed an obscure ~150 year-old publication, so that's cool :) On the other hand, this should really be open access at this point, so that's lame :( SemanticMantis (talk) 17:10, 2 September 2015 (UTC)[reply]

Thanks, that was all I needed in this case. Yes, it is indeed a shame that Elsevier want $35 for what's now a public domain work. Rjwilmsi 07:59, 3 September 2015 (UTC)[reply]

Which optical illusion is this?[edit]

We have this circular carpet in our house:

http://sjbaker.org/CarpetIllusion.jpg

...and on some occasions (but far from all) when I stare at it from about the angle of the photograph, the red petals of the flower design seem to flow inwards across the white region towards the brown center. The subjective rate of flow seems quite fast - maybe one centimeter per second...yet (of course) it never reaches the center. A very strange illusion because it's very obvious that nothing is moving...yet the impression that it is moving is incredibly strong and doesn't go away if I blink or look away and look back again. It only happens quite rarely, but it's happened several times now, so it's repeatable - I can't associate it with any particular mental state, or even particular lighting conditions.

Our articles Optical illusion and List of optical illusions don't mention much about illusions that produce a feeling of motion. Illusory motion has a couple of examples that seem kinda similar...but no decent explanations:

Any ideas why my carpet design (which seems to have none of the attributes from any of the example images) might be doing this?

SteveBaker (talk) 18:53, 2 September 2015 (UTC)[reply]

Normally such illusions are caused by conflicting inputs from two different pathways in the cortical lobe's visual system. You might want to mention it to your general practitioner, because there can be medical reasons for such symptoms. μηδείς (talk) 19:28, 2 September 2015 (UTC)[reply]

Clarification - do you get the illusion from your posted photo, or only in real life viewing? As for potential explanations - see the refs in Peripheral_drift_illusion (which you should probably add to illusory motion), particularly the works of Akiyoshi_Kitaoka (and his personal gallery [5]), which go into a lot of neuropsych detail. I would say the features your carpet shares with some of those examples are blobs of color with borders of different colors, and the border thickness varies radially. As for the variability of your perceived illusion- I'll assume that LSD intake is not a variable that you have not noticed, but I think (WP:OR) lighting and fatigue can play a role in strength of perceived illusion, even if not registered consciously (here's a few slightly relevant refs [6] [7] for that claim, but I'll leave the OR flag for now). SemanticMantis (talk) 19:47, 2 September 2015 (UTC)[reply]

I've heard the term scintillation used to describe such optical illusions. We don't have a specific article about that usage, but it is mentioned at grid illusion aand at the Scintillation dab page.--Jayron 32 20:32, 2 September 2015 (UTC)[reply]

I've only noticed the illusion in real life - but it only happens fairly rarely (like maybe once a week) - and I don't look at the photo that often. I don't do drugs of any kind - nor alcohol more than one or two units per week - and I'm not under medication or sick. Fatigue is a possibility - but not excessively so. SteveBaker (talk) 20:35, 2 September 2015 (UTC)[reply]

(not an answer, just a random related anecdote) There was a very common sort of mattress some time ago with a pattern of parallel blue lines on it. I have always perceived shifting jagged yellow lines running at roughly 60 degree angles to these when seeing that pattern. But for some reason I've never duplicated it on a computer monitor... Wnt (talk) 22:04, 2 September 2015 (UTC)[reply]

Anecdote 2, I see very strong 'warping' of the pattern in the blue and green example shewn above. I wonder if some form of color related vision defect, or an astigmatism, somehow mapped by eye movement is involved? Greglocock (talk) 07:20, 3 September 2015 (UTC)[reply]

I think everyone sees warping in that blue/green pattern. I see it all the time and can't make it go away no matter how I look or concentrate on it. The fact that everyone sees that effect means that it can't be due to astigmatism. I do, however have astigmatism (more so in one eye than the other) - so that might relate to the original carpet "illusion". I don't quite see how it would explain it - but that's definitely a possibility. SteveBaker (talk) 18:16, 3 September 2015 (UTC)[reply]

Long shot, Steve, but I (and I assume everybody else) experience a similar illusion after staring at approaching/diverging objects for a period, as when driving or looking out of a train window facing forward. Does your occasional illusion occur after you've just driven home? {The poster formerly known as 87.81.230.195} 185.74.232.130 (talk) 13:16, 3 September 2015 (UTC)[reply]

No, certainly not that. I experienced it again just before posting the original question and I hadn't been out of the house all day. The impression does fall somewhere between 'flow' of the red color and as if the object were getting closer to me and growing due to perspective - so I get what you're saying about approaching/diverging objects. SteveBaker (talk) 18:16, 3 September 2015 (UTC)[reply]

Unknown watch[edit]

Hello guys. My father bought an unknown watch couple of years ago. Handwritten sticker on it says its from 1920. Nobody has any idea what this thing is good for. I guess that this is some device from the military. Maybe some military history experts here are able to explain what kind of watch this is and what it was used for. The following words i found on the backside of the watch: Charles Churchill co limited london 723317 DRP 88992 +100230 CI-Dessouse Fornitures DEPE Charge Hieronder ReservedeC (not sure about the last letters)

Here are pictures i made from the front and backside of the watch.

If anyone knows what this is it would be nice to know the offical name of this thing.

--Onegermanguy (talk) 21:18, 2 September 2015 (UTC)[reply]

The The Churchill Machine Tool Company made pocket watches around the start of the 20th century, though this aspect of the business is not mentioned in our article. I can find plenty of records of them going up for auction for 100-200 British Pounds, depending on age and state of preservation. I haven't been able to figure out how you would tell which specific model you have. The company still exists, so perhaps you could contact them: [8]. Someguy1221 (talk) 21:39, 2 September 2015 (UTC)[reply]

It's a stopwatch. The various numbers are, initially, confusing. This image of a different Charles Churchill mechanical stopwatch, and labels the ring that has 60 at the top as "production per hour" - so I guess a factory foreman would time the duration of one operation in the factory, and if that took 30 seconds, that ring would tell him the line would produce 120 items per hour. That's the red ring on your watch, and the outer black ring on one I linked to. -- Finlay McWalterᚠTalk 21:47, 2 September 2015 (UTC)[reply]

Looking at it more, the other coloured rings are much the same. The small hand is a minute counter. It's labelled with colours, so the watch can compute productions per hour if the time is 0-1 minute (the red ring), 1-2 minutes (the black ring), and 2-3 minutes (the blue ring). So if the foreman timed an operation and it took 2 minutes 30 seconds, the big hand would be facing down (the 5) and the small hand would be in the blue segment. So the production line would be producing 24 items per hour (which is 60 / (2 and a half). -- Finlay McWalterᚠTalk 21:54, 2 September 2015 (UTC)[reply]

I still don't understand the outermost (0-9) ring of black numbers. All I can think of is that the gejigger on the top-left is a mode selector - that it allows "standard mode", where the big hand takes 60 seconds to go round, and "fast mode", where the big hand takes only 10 seconds. That would allow timing of fast operations. -- Finlay McWalterᚠTalk 22:03, 2 September 2015 (UTC)[reply]

As a railfan, I sometimes like to measure the speed of trains by timing the mileposts (or kilometer posts) as they go by. This stopwatch would be perfect for that purpose if, as I suppose, the big hand takes 1 minute to go around and the little hand takes 30 minutes. The watch gives a direct reading in decimal fractions of a minute. In the photo the little hand has not yet advanced from the 10 to the 11; the big hand has passed the 7 on the outer scale (10ths of a minute) and 7 marks of the fine scale (100ths of a minute), and is about 20% of the way to the next mark. So the time it's showing is 10.772 minutes. The concentric red, black, and blue scales inside the inner scale are parts of a reciprocal scale, giving the speed per hour directly if the time is between 0 and 3 minutes. For times under 1 minute, you use the red scale; for 1–2 minutes, the black; for 2–3 minutes, the blue. That's what the three colored wedges on the scale for the little hand mean. For example, if it takes 0.3 minutes to cover a mile, your speed is 200 mph (that's 60/0.3, where 60 is the number of minutes in an hour); 1.3 minutes is 60/1.3 = 46.1 mph (they say, though 46.2 would be closer) and 2.3 minutes is 60/2.3 = 26.0 mph. Or it could be kilometers and km/h (or, as hinted above, the time to produce one unit and the number of units produced per hour.) --65.94.50.17 (talk) 04:00, 3 September 2015 (UTC)[reply]

So that's how Ozzie Nock & Co used to do it! {The poster formerly known as 87.81.230.195} 185.74.232.130 (talk) 13:21, 3 September 2015 (UTC)[reply]

Actually, in Britain the railways are mileposted every quarter mile, so they'd typically time that distance. If they timed longer intervals, we might have a clearer idea of whether City of Truro really exceeded 100 mph that day.--65.94.50.17 (talk) 04:23, 4 September 2015 (UTC)[reply]

Thanks. The explaination with that it is a stop watch for the foreman sounds real legit. But i will write the company. Thanks for the hint that they still exist and the contact. --87.140.193.0 (talk) 09:16, 3 September 2015 (UTC)[reply]

Stem cell[edit]

"Stem cells are undifferentiated biological cells that can differentiate into specialized cells and can divide..." I cannot understand this sentence. It says two contradicting things at the same time. If stem cells are undifferentiated cells then how can it differentiate? Can someone explain? Thanks! 146.151.96.180 (talk) 22:17, 2 September 2015 (UTC)[reply]

"undifferentiated" means "not differentiated yet", not "can't ever differentiate". -- Finlay McWalterᚠTalk 22:20, 2 September 2015 (UTC)[reply]

Right. A slightly more self-explanatory term is pluripotent - having to power to do many things. Embryonic stem cells are completely pluripotent and can eventually differentiate into any cell type, while e.g. Keratinocyte#Cell_differentiation are only mildly pluripotent, and can only differentiate into a few different things. SemanticMantis (talk) 23:39, 2 September 2015 (UTC)[reply]

It's a bit like saying "my baby isn't an architect". Well, no, because he's a baby right now. Some day, with the proper training, he could be an architect. He could also be a lawyer, a doctor, or a plumber, depending on what happens in the intervening time. Think of stem cells like that. They aren't anything right now, but could be any number of things, given the proper "training" and time. --Jayron 32 00:19, 3 September 2015 (UTC)[reply]