Wikipedia:Reference desk/Archives/Science/2011 November 28

= November 28 =

Nuclear grenade
Could one of the elements with a lower critical mass (e.g. curium, californium) be used to make a nuclear grenade? --70.250.212.95 (talk) 01:07, 28 November 2011 (UTC)


 * Not a hand thrown grenade. The local neutron flux alone would kill the thrower.--Aspro (talk) 01:34, 28 November 2011 (UTC)


 * The OP may be interested in Tactical nuclear weapons. -- Jayron  32  02:04, 28 November 2011 (UTC)


 * I don't think there would be much use for a grenade that (1) will be WAY too heavy (and bulky) to either throw by hand OR to launch using any existing grenade launchers, (2) will have an explosive yield that is at the VERY LEAST equivalent to an eighteen-wheeler completely filled with dynamite when a tiny fraction of this explosive power will do for a grenade's primary application, and (3) because of this ginormous explosive power, will most likely kill any person attempting to use it, as well as any other friendly troops in the vicinity. 67.169.177.176 (talk) 06:08, 28 November 2011 (UTC)


 * I can't see on what basis you are asserting that there is a minimum yield for a super-critical nuclear reaction. — Preceding unsigned comment added by 129.67.38.22 (talk) 11:38, 28 November 2011 (UTC)
 * Because, as you say, it needs to be super-critical. That means you need at least the critical mass. That means the minimum yield you can get is the yield from a critical mass, which is still very high. --Tango (talk) 13:12, 28 November 2011 (UTC)


 * Critical masses of fissile materials in simple configurations are all of the order of several kg or greater. Even if you can reduce the critical mass to 1 kg, you still have (a) a very heavy grenade and (b) a device with a yield of at least 1011 J i.e. the equivalent of about 25 tons of TNT. The smallest US nuclear warhead ever developed had a yield equivalent to 10-20 tons of TNT - enough to destroy a couple of city blocks. Since smaller and less powerful nuclear devices would have had plenty of military and civil applications, I imagine they would certainly have been developed if it were feasible. Gandalf61 (talk) 13:12, 28 November 2011 (UTC)
 * I'm not sure what applications they would have, actually. In reality, the only application of a nuclear weapon is as a deterrent (if you actually end up using the thing, you're dead anyway and are just trying to take the other guy down with you). For a deterrent, I'd say the bigger the better (but delivery systems are the important thing, really). --Tango (talk) 13:25, 28 November 2011 (UTC)
 * The development of different types of tactical nuclear weapon shows that many people believed nuclear weapons could be used for purposes other than threatening total destruction: proposed uses include for penetrating deep bunkers, other demolition e.g. to block enemy transport routes, nuclear mines, and in place of regular artillery. Biological and chemical weapons are also nearly as dangerous for the users as for the victims, but that hasn't stopped countries developing and even occasionally using them. --Colapeninsula (talk) 14:27, 28 November 2011 (UTC)


 * For civil applications see Operation Plowshare, Project Orion and Nuclear Explosions for the National Economy. The idea of using nuclear detonations in civil engineering projects and to launch spacecraft seems dangerous and bizarre now, but it was very seriously considered and studied in the 60s. Gandalf61 (talk) 14:48, 28 November 2011 (UTC)
 * In Paranoia, The Computer will provide its operatives with thermonuclear hand grenades. Of course, the kill radius of a thermonuclear hand grenade is significantly greater than the deployment radius... --Stephan Schulz (talk) 15:15, 28 November 2011 (UTC)
 * You don't get seven respawns in real-world warfare. I can't believe anyone still plays that game anymore... 67.169.177.176 (talk) 01:44, 29 November 2011 (UTC)


 * I don't know where all this baloney comes from about minimum yield but I have come across it else where before. The skill in bomb design is to get a good yield before the core flies too far apart for the runaway chain reaction to continue. Even in the definition of 'one point safe' the yield is not to be over 4 lb equivalent  TNT - hardly a truck load. A tucker  probably needs  more Budweiser than that the night before -for the day ahead.   Its more down to the point of why use a very expensive weapon where the yield is only a few multiples of the explosive forces required to compress the core to super criticality. Its just not economically feasible.--Aspro (talk) 15:47, 28 November 2011 (UTC)
 * Indeed&mdash;our article criticality accident lists any number of accidental creations of supercritical masses that were associated with negligible or nonexistent blasts. (Even the cases that did involve explosions were generally caused by high-pressure steam, rather than a violent explosion of the radioactive material itself.)  Many of these non-explosions were nevertheless quite deadly to those in the vicinity due to the extraordinarily intense burst of ionizing radiation, but the military already has much faster and less costly ways to kills individual roomfulls of people.
 * That said, I don't doubt that there are serious engineering challenges involved in 'tuning' the output of a low-yield device; getting the precisely-correct miniscule fraction of the total critical mass to fission (not too much, and not too little) is probably kind of hairy. And there's no need for such low-yield devices to be nuclear&mdash;there are tested delivery systems already in place for putting a ton or less of high explosive down on a battlefield. TenOfAllTrades(talk) 17:22, 28 November 2011 (UTC)


 * A quick Google around for something old, yielded this (if you pardon the pun). It is dated  1978.  All this business about the minimum core mass yielding multiples of kilotons has more to do with the limitations of early nuclear devices than that which is physically achievable.--Aspro (talk) 18:00, 28 November 2011 (UTC)
 * This reminds of the FURY/TNW from the game Halo - a nuclear mine that is either remotely detonated or time-delayed. It is described as being of slightly under one megaton yield, with a blast radius of approximately 1.2 kilometres. It is not designed to be thrown as a grenade, it is to heavy to be thrown far enough unassisted by a human. However, with the use of a MJOLNIR/PAA, itself nuclear powered, it can easily be thrown two kilometres. The FURY has rarely been used in this role, as the MJOLNIR is itself a rare item. Plasmic Physics (talk) 11:42, 29 November 2011 (UTC)


 * There is also the problem of the EMP causing damage to the powered assault armour, unless the thrower manages to take cover behind EMP opaque barier like solid rock. Should the MJOLNIR be caught in the EMP, the system is forced into a reboot state which takes several minutes trapping the wearer in a giant paper weight. The MJOLNIR weighs approximately 400 kilograms, and has enough strength to easily lift a small car clear off the ground. Plasmic Physics (talk) 11:56, 29 November 2011 (UTC)


 * Excuse me, what's that Mjolnir/PAA thing? Is it something like the Titan armored walker from Tiberian Sun? 67.169.177.176 (talk) 06:43, 30 November 2011 (UTC)


 * It is a type of nuclear powered assault armour, among its attributes it increases the wearer's strength, and reaction rate, and obviously provides a level of protection from small-arms fire. Plasmic Physics (talk) 08:51, 30 November 2011 (UTC)


 * Thanks for the info. I'm a Command & Conquer fan, but I don't play Halo. 67.169.177.176 (talk) 02:50, 1 December 2011 (UTC)


 * The M67 grenade weighs 14 ounces. The truly remarkable table at Critical mass lists Cf-252 as requiring a 2.73 kilogram spherical mass - but perhaps wrapping it in beryllium or some other neutron reflector can reduce this?  Making a nuke that is as handy to throw as a hand grenade is obviously a difficult technical feat ... but proving it to be impossible might be even more difficult.  Suggestion: longer delay fuse!  Just a thought. ;) Wnt (talk) 19:52, 30 November 2011 (UTC)
 * Problem with your suggestion: This will give the enemy more time to throw that thing back at you! :-) 67.169.177.176 (talk) 02:46, 1 December 2011 (UTC)
 * Well, hopefully, by that point you've already told the cabbie you meant the other Republican National Convention, the one out in the suburbs, and step on it! Wnt (talk) 04:43, 1 December 2011 (UTC)
 * And what makes you think I'd make the Republican convention a primary target (as opposed to, say, the Ground Zero mosque)? 67.169.177.176 (talk) 05:31, 1 December 2011 (UTC)
 * Well that would just be silly! B^) Wnt (talk) 17:06, 1 December 2011 (UTC)

Using pulleys to open a coop door
I have a chicken coop and this motor. The door on my coop swings open from the top. Instead of adding side channels and converting the door to open by sliding up and down, I'd like to keep it a swing type door and just use a pulley. I'd also like to increase the weight of the door a bit. How could I use pulleys to be able to lift more weight? And does anyone see any major flaws in my idea? Dismas |(talk) 01:40, 28 November 2011 (UTC)


 * I think everything you need to know is in the article pulley, where you can see how to design a compound pulley to get a mechanical advantage and lift a heavier weight. One possible problem is that the motor doesn't pull for long enough - the trade-off for lifting more weight is that you have to pull for longer, in exactly the same way that vehicles in lower gears move more slowly, so more rope will be used up. It's not clear to me whether "swings open from the top" means the door swings from the top (and hinges at the top) or opens from the top (and hinges at the bottom). If the latter, you'll need some component located below the level of the door, which might be awkward to secure in place and might lead to unwanted interference between the mechanism and the chickens. Card Zero  (talk) 03:56, 28 November 2011 (UTC)


 * The hinges are on the top of the door. Dismas |(talk) 03:59, 28 November 2011 (UTC)


 * Seems fine, then. You could hammer in some simple U-shaped guide for the rope (or cord, or whatever it is) somewhere above the hinge, to ensure it's still travelling straight vertical when it reaches the motor, in case that makes any difference - it might not like pulling at an angle. Nothing else springs to mind, apart from that something unanticipated always goes wrong ... huh, I see that page you linked specifically mentions converting hinged doors to sliding ones, I'm not sure why. I can't see what difference it makes, except perhaps that the motor might fail due to having to work at a non-constant rate (would that even be true?) ... I'm going to read the manual. Card Zero  (talk) 04:05, 28 November 2011 (UTC)


 * The manual is no help ("IF YOUR DOOR IS A SWING-OPEN TYPE, FIRST CONVERT IT TO VERTICAL LIFT-LOWER", it says, without further explanation), but I suppose the problem is that the first half-inch or so of pulling a swing-open door (in the orientation in question) will be very difficult. I can't quite explain why this is: it may be equivalent to working in a very high gear (because the door is a lever - can a lever be arranged so as to make work harder? - oh yes, of course it can) or there may be something else going on, like friction . Pulling from a different point, which is not located directly above the door, seems likely to help - a point both above the door and some distance in front of it. You could have a stick with a wheel on the end projecting out from above the door, and run the cord over that wheel. (Uh, unless it opens inwards, in which case of course I mean some distance behind not in front.) Card Zero  (talk) 04:30, 28 November 2011 (UTC)


 * Agreed. Note that this means the door won't open quite all the way, but I imagine anything past horizontal is fine.  You might want to use bungee cords instead of rope, as the elastic might even out the differences in force needed at different relative angles to the door as it opens. StuRat (talk) 05:01, 28 November 2011 (UTC)


 * Bungee cord isn't an option. The cord would never fit. And the manual specifies using 80 or 100# fishing line. Dismas |(talk) 05:09, 28 November 2011 (UTC)


 * NEVER REPLACE LIFT-CABLE WITH METAL CABLE OR NYLON CORD BECAUSE MOTOR DAMAGE WILL OCCUR! I'm not sure why it says this. It also says "Lift-Cable can stretch". I think perhaps the deal is that the lift-cable is designed to stretch - this might be a crude way of avoiding disasters when something jams. The cable is the intended failure mode. Card Zero  (talk) 05:13, 28 November 2011 (UTC)


 * OK, then, it sounds like it's already sufficiently elastic for the purpose. StuRat (talk) 05:20, 28 November 2011 (UTC)


 * I'm wondering now whether putting a section of elastic, or a stretchy spring, or rubber band, at the end of the cable where it attaches to the door, might do the job, and allow the motor to pull from directly above - since the first work it would do would always be to stretch the spring, which wouldn't be all that difficult, so the motor wouldn't burn out or jam. Card Zero  (talk) 05:23, 28 November 2011 (UTC)


 * It's worth bearing in mind that any door could be counterbalanced - a door hinged at the top possibly being simpler than a sliding door in this respect. I think what you need though is a pulley wheel attached to the door hinge axis, so regardless of where the door is, the leverage is about the same. One thing you need to consider though is whether the door is just there to 'fill the hole', or if it is there to stop chickens getting out - or foxes getting in. A vertically-sliding door is going to be harder to open when 'shut' by your motor than a hinged one. I think chickens have just enough sense to get out of a closed door by pushing on it, and I'm sure that any fox can open a door that opens towards it, given the incentive. AndyTheGrump (talk) 05:24, 28 November 2011 (UTC)
 * "No counter-weight needed" is one of the selling points of this device. It's a cunning plan to add one, though. There's nothing to be gained by attaching the wheel (or cable) near to the hinge - that's essentially the same problem as pulling from directly above while the door is closed; it's the worst possible point to pull from (unless your aim is to move the door as fast as possible while expending as much energy as possible). However, just making the pulley multiply force a lot is another possibility. That way the door creaks open painfully slowly, and all the chickens pile up behind it trying to shove through the gap (which probably helps), and the motor survives the process. Card Zero  (talk) 05:33, 28 November 2011 (UTC)


 * The reason I want to use pulleys to increase the advantage is because I want to add more weight to it and therefore make it harder for critters to lift it. And I'm not concerned with the chickens opening it on their own. I'll time it to open around sunrise and close around sunset. The birds are already in the coop by the time sunset rolls around, if you're thinking they may get locked out. And they're pretty active at sunrise. Dismas |(talk) 05:34, 28 November 2011 (UTC)
 * I have a bad feeling that it won't be possible to set the motor to pull for as long as you'd like it to, in order to lift a really heavy door (slowly). The way this thing works is by moving "actuators" around, which are like little fingers that operate the on and off switch. "User adjusts Actuators closer to, or farther away from Stop Switch to set desired open and closed stop positions." They are positioned around a wheel, so the full circumference of this wheel equates to the longest pull time. I don't know how long that is, but in one of the picture they occupy a quarter of it, so it looks like you can have a maximum of four times the travel of a typical sliding door. I guess you'll want all of that extra length, and the mechanical advantage of the pulley, to go into making the door heavier; so finding or building a more advantageous point to pull from seems essential. Card Zero  (talk) 05:54, 28 November 2011 (UTC)
 * I don't have the motor here with me. I wish I did.  So, I can't see where these 'Actuators' are.  But the instructions also say "Motor can be operated by AC timer switch, remote control, and X-10 control (control not furnished)."  I have an Insteon controller that, unless I'm quite mistaken, I can program to turn the motor on for X number of seconds and then turn off, regardless of where these actuators are positioned on the wheel.  Sorry for not pointing that out sooner but I didn't think it would come up.  Dismas |(talk) 06:04, 28 November 2011 (UTC)
 * That's great! You might have to mutilate the door-opener with pliers or something to get the actuators off, then. Hopefully they can just unscrew or something - or you could do something with the wiring to bypass the off switch. This is excellent, though - now you're limited only by how much space there is to physically contain the pulley system. The door can weigh half a ton and open in a geological timeframe. (I might question, with all this customization, why you need to buy a fancy-pants $87 motor. You're adding to its gearbox, with the pulley, and you're not using its timer or auto-off facility. Just salvage a low-voltage motor from somewhere and use that, surely? ... Oh wait you said you already have the motor. Carry on.) Card Zero  (talk) 06:10, 28 November 2011 (UTC)
 * Hmmm, what bothers me about this scenario is that the wind pushing on a swinging door should push directly against the motor. If the motor isn't absolutely locked in place against such pressure, the door should end up at some odd position; then when the motor automatically closes it, perhaps it's partly open, or perhaps it is fully closed while the motor is still running.  By comparison a door that drops down from above should get very little pressure up or down from gusts of wind.  Does that make any sense? Wnt (talk) 20:00, 30 November 2011 (UTC)

Diluted Alcohol
How many parts (drinking) alcohol would it take per parts water to keep the water sterile for (say) a month in an emergency situation? Assume 95% grain alcohol is being used. I know there are probably more cost effective ways of keeping water purified, but I'm just interested in the ratio. --68.190.114.151 (talk) 11:55, 28 November 2011 (UTC)
 * If the water is sterile to start with, then you just need to keep it in a sealed container. --Tango (talk) 13:20, 28 November 2011 (UTC)


 * Like I said, I'm interested in the ratio, not the practicality. And if the container doesn't also keep light out, photosynthesizing microbes will grow in it. Water will not necessarily stay safe unless it's kept in complete darkness and free of nutrients. Yeast brings concentrations up to as high as 22%, but that's probably much higher than necessary. Perhaps I should rephrase, but I think the information I'm after is clearly expressed. --68.190.114.151 (talk) 14:13, 28 November 2011 (UTC)


 * Er...not really. As Tango says, if you start with sterile water in sealed, sterile containers, then you won't get any growth even at 0% ethanol because there isn't anything there to grow.  (Completely airtight containers of clean water are generally pretty resistant to growing things too, because they don't contain a carbon source.)  If you're trying to permanently kill any microorganisms in already-contaminated water, even straight-up ethanol won't be completely effective; some types of bacterial spores are quite capable of surviving for years (possibly indefinitely) in ethanol (see also Ethanol and the reference there).  This textbook notes that ethanol is bacteriostatic (stops bacterial growth and spore germination) at around 10%, and is increasingly bactericidal at 30% and up&mdash;though efficacy will depend on the specific bacterial species.
 * The concentrations required to inactivate viruses and kill yeasts vary depending on concentration and exposure time, but seem to be well above 20%.  (Be aware most studies of the effect of ethanol concentration on pathogen survival are aimed at sterilization of medical instruments and food handling equipment, so tend to look at exposures of 5-60 minutes rather than days or weeks.)  TenOfAllTrades(talk) 15:44, 28 November 2011 (UTC)


 * In medieval times, everyone (even the children) had to drink fermented beverages like beer or mead because water was often unsafe to drink due to bacterial contamination and/or in short supply (e.g. during a typical siege). This led to everyone being in a constant state of slight inebriation all the time, from breakfast until bedtime, which could explain some of the ill-considered decisions made both by actual historic characters in that time period, and also by fictional characters in that period's literature. 67.169.177.176 (talk) 01:13, 29 November 2011 (UTC)


 * Thank you, TenOfAll, that's the information I was looking for. I appreciate it. --68.190.114.151 (talk) 16:11, 28 November 2011 (UTC)


 * Most wild yeasts will be killed by 10% ethanol. Standard wine is 13% ethanol by volume, and it takes carefully selected strains to be able to ferment the wine any higher than that (which is why most alcoholic drinks above 13% are distilled or fortified with distilled ethanol.)
 * Hopped beer will not spoil for a month or two if it is kept in a sealed container, even if it is not brewed in the perfectly sterile conditions, at as low as 5%.--Itinerant1 (talk) 00:20, 30 November 2011 (UTC)

physics
sorry for the title :D. i don't wanna be misleaded, so can [] be true?why?--Irrational number (talk) 19:34, 28 November 2011 (UTC)


 * If you wonder why you have not received an answer, it might be because you did not frame an answerable question here. I am loath to click on links placed here, and I may not be alone in that.  So, you may get better results if you clearly state your question in words.  If an off-WP link is essential, then please describe what viewers will see.  I hope this is helpful. -- Scray (talk) 04:17, 29 November 2011 (UTC)


 * um... okay this is some guy explaining something he calls super ferrofluids, he says some thing like that, for example, some ferromagnetic particles mixed in a super fluid (which moves frictionlessly), will continue accelerating (or speeding up) in presence of a magnetic field, reaching relativistic speeds, warping spacetime and may be usable for interstellar travel, if I understood it well, now, is such a thing possible? Im skeptical to what he says because he seems to believe in UFOs and stuff like that... (I'm the OP)--95.82.51.215 (talk) 04:32, 29 November 2011 (UTC)


 * Thanks for explaining. Like Scray, I don't usually click on links here.  I'm not an expert on superfluids, but a simple energy argument shows that the theory has flaws.  The energy for the high speed particles has to come from the magnetic field, and presumably this has a limited energy input, so will have a smaller and smaller accelerating effect as the speed of the ferroparticles approaches the speed of light.  It is unlikely that sufficient energy will be available in the field to create a significant gravitational effect to warp spacetime, and even if an enormous amount of energy is fed in, the effect will be only that of a large mass equivalent to the energy fed in.  Perhaps an expert on general relativity can clarify further.    D b f i r s   09:05, 29 November 2011 (UTC)


 * ... or an expert on materials and superfluids (see below). Thanks Brains, I hadn't watched the video!    D b f i r s   18:37, 29 November 2011 (UTC)


 * Full disclosure: In my opinion, anyone who calls crackpots "courageous" for having spoken up against some mysterious oppressing force is considered a crackpot in my mind.
 * There are many things he mentions in passing which I expect he doesn't understand, and several things he says which don't make sense. It would take multiple PhD's to understand all the fields he brings up, but I will just point out the things that I know off the top of my head as being problems (note that when I say "impossible", what I really mean is "highly, HIGHLY unlikely"):
 * A superfluid other than Helium-3 and Helium-4 at workable scales is impossible. The video speculates that an antigravity effect could be achieved with a substance that was both a superfluid and a ferrofluid, and helium is not a ferrofluid. Because of the complicated nature of quantum mechanical problems such as this (I'm not sure science even HAS an answer to this as of yet), but I would expect that any theoretical superfluids more complicated than helium would need to be cooled to exponentially lower temperatures than the current 2-4 K needed to achieve superfluidity in Helium, and that is just not practical even in the distant future.
 * A "mercury-based plasma" which is "supercooled" is impossible on the scale described. Plasmas can only be achieved at very high temperatures, or under extremely carefully prepared laboratory experiments. No one refers to any suspension of solid particles in a liquid other than the blood-related one as "plasma", so his disclaimer is moot.
 * He seems to think that you could somehow combine a superfluid and a ferrofluid to get this effect. This is not true: superfluids are ridiculously sensitive to disturbance; any contamination or mixing would destroy the needed properties.
 * Mercury is, by a large margin, the metal with the lowest freezing point. There is no way to use some sort of other metal to create an alloy, since any mercuric alloy would necessarily have a higher freezing point, and nothing else can dissolve in mercury (this point I'm actually not QUITE sure about, I'd like some independent confirmation from a chemist), there is no way to get mercury to stay liquid at 150K.
 * The fact that he thinks, not only that this is possible, but that his Youtube audience may be able to make some headway where researchers with million-dollar budgets have failed, suggests that he might not have the best grip on reality.
 * I usually try not to seem overly critical of budding scientists, but arrogant conspiracy-theorists are half the reason why the public doesn't trust the 99.9% of scientists who are working to better humanity. This guy can be safely ignored unless he changes his "the THEY are keeping this technology out of OUR hands" tune.- Running On Brains (talk) 18:21, 29 November 2011 (UTC)
 * 4. isn't true - it is common for alloys to melt at temperatures lower than either component (NaK, for example, melts at a lower point than either potassium or sodium). In particular, Thallium-Mercury alloys melt as low as -58oC, (20 degrees colder than pure mercury), and are used in low temperature thermometers.  Buddy431 (talk) 01:45, 30 November 2011 (UTC)
 * Thanks Buddy. - Running On Brains (talk) 07:48, 1 December 2011 (UTC)
 * And don't forget to mention the fact that anti-gravity is impossible according to the Equivalence principle. Dauto (talk) 02:17, 30 November 2011 (UTC)


 * Unimpeded inferences:


 * Just in the first half-minute of the video, I thought of electromagnetic propulsion, and a magnetic (dipole?) torus must have weird effects as well. I've heard a claim that gravitational field lines are generated at right angles to the electromagnetic plane when a strong electric and magnetic field are intersected at right angles, though am doubtful about its validity, though it discussed zero-point energy. Maybe UFOs if they exist use quantum tunnelling. Quantum levitation works too, but it requires very cold temperatures capable of sustaining superconductors, temperatures only available in deep space. Many theories for gravity exist, including gravitons. One word in the video was gravitomagnetic (and spacetime manifold), but so far scientists have not found a unifying theory. Free energy is likely impossible unless one could synthesize antimatter using no energy. Based on the images of spherical droplets, the lotus effect (hydrophobic effect of water beading) comes to mind. The only minimally-plausible design seems to involve an impenetrable barrier separating near-absolute-zero superconductors from high-temperature-pressure plasma fluids within a spinning torus-shaped centrifuge. E-folding topological defects in Spacetime wormholes? Fractal tesla coils? ~ AH1 (discuss!) 02:04, 1 December 2011 (UTC)

Is there a transparent rock tumbler anywhere in existence?
One of my best friends asked me to get him a see-thru rock tumbler for Christmas back in 2003 or 2004. Even though I told him that I could not find any online, I never completely let it go. It may be invented by now, or what was an even more obscure source back in those days may not be as much now.

Therefore, is there a see-thru rock tumbler anywhere? (It seems that Google beats around the bush on this one. Maybe it uses an esoteric set of keywords?) Hopefully I can finally put this matter to rest. Thanks. --129.130.212.185 (talk) 19:52, 28 November 2011 (UTC)


 * I suspect that the problem is that most materials wouldn't stay clear in that environment. Millions of tiny scratches would cloud up plastic and probably even glass.
 * You might find a vibrating one instead of a rotating one, though. This one here might work. APL (talk) 20:16, 28 November 2011 (UTC)


 * What about CVD diamond coated corundum? Plasmic Physics (talk) 22:05, 28 November 2011 (UTC)


 * Links for the curious: CVD diamond, corundum. -- 203.82.66.204 (talk) 23:42, 28 November 2011 (UTC)
 * Synthetic diamond is an amazing invention. Something alchemists have been looking for for countless �centuries, but I don't think they make rock tumblers out of it, yet. APL (talk) 23:50, 28 November 2011 (UTC)


 * No, no, no. A rock tumbler made from the much softer and less resource-expensive corundum, coated in a thin layer of CVD diamond. Plasmic Physics (talk) 11:22, 29 November 2011 (UTC)


 * Here's one: . I'm pretty sure there is no diamond or corundum involved, though one does have to wonder about the propensity for scratches.  Dragons flight (talk) 11:36, 29 November 2011 (UTC)


 * It's probably polycarbonate. --Colapeninsula (talk) 17:15, 29 November 2011 (UTC)
 * 220v? That's odd. It's being sold on the American version of Amazon, but it needs european electricity. If you're in USA you're going to need a gizmo like this to convert from your American 110 to european 220.  (One of those plug-adapters alone won't do it. You need an actual voltage converter.) APL (talk) 20:20, 29 November 2011 (UTC)
 * American houses typically have a few 220 volt outlets for things like ovens, refrigerators, and air conditioners. (That last is a hazard - I once lost some electronics to an apartment with a 220 volt outlet for an air conditioner (I'm told) that was exactly like a regular 110 volt outlet except things plugged into it died instantly!) Wnt (talk) 21:33, 29 November 2011 (UTC)
 * That sounds like it was a jury-rigged job done by a rental property owner who was too cheap to hire a proper electrician or to buy the correct parts. It's definitely a violation of whatever local electrical code applies to connect a 220V supply to a 110V (NEMA 5-15) receptacle.  Similarly, there's no way that a 220V appliance would be sold with a 110V plug; someone no doubt did a bit of DIY repair and couldn't be bothered to get the correct replacement plug.  If your landlord hadn't been doing something illegal, it wouldn't be hazardous&mdash;it's not physically possible to fit any standard North American 110V plug into a 220V receptacle, or a 220V plug into a 110V receptacle. TenOfAllTrades(talk) 01:55, 30 November 2011 (UTC)

Correct Speed?
The speedo on my Honda CRV reads about 2mph faster than the reading on my iPhone, which is the most accurate please?--85.211.153.242 (talk) 20:07, 28 November 2011 (UTC)
 * I would trust the GPS on your phone. Car speedometers aren't really very accurate. See Speedometer
 * APL (talk) 20:19, 28 November 2011 (UTC)
 * Yes, GPS is much more accurate on a straight level road with a good signal. Your speedometer just counts revolutions of your wheel, and guesses at the effective radius of the tyre.  It is set to over-estimate for a worn tyre and heavy load, so that it doesn't get you into trouble with the law by reading low for new tyres and a light load.  The discrepancy is likely to be a fixed percentage of the speed (more than 5% high in some cases for old tyres and a loaded vehicle).    D b f i r s   22:02, 28 November 2011 (UTC)
 * Agreed. When I put bigger tires on my Jeep, my speedo was about 6 mph off.  I had to account for it all the time.  Dismas |(talk) 00:16, 29 November 2011 (UTC)
 * Note that the Honda's inaccuracy is likely doing you a favor. Most cars overestimate your speed, and that helps you avoid getting a ticket since you are generally going a little slower than you think you are. Mingmingla (talk) 18:13, 29 November 2011 (UTC)
 * Car manufacturers have an incentive to have the speedometer read fast, since you think you are getting more speed for a given gas mileage, as well as faster acceleration. Perhaps they are doing you and the driving public a favor by reducing the frequency and severity of accidents. At the same time, they are doing themselves a large favor by making any mileage based warranties expire sooner than they rightfully should have. There have been lawsuits over this.,  Edison (talk) 21:39, 29 November 2011 (UTC)
 * According to the article I linked earlier, in many nations they have a legal requirement that the speedometer never read low. So of course they add a couple of percent to the readout to give themselves a margin of error. APL (talk) 23:56, 29 November 2011 (UTC)
 * I wouldn't be surprised if even in countries which don't have such a law someone would think of suing a manufacturer if they got a speeding ticket even though the speedometer read a legal speed. -- Q Chris (talk) 11:22, 30 November 2011 (UTC)
 * In Pennsylvania, at least, it's not uncommon to find a radar speed reader with a visible display set up just before construction zones which displays your speed as you drive toward it. These are great for purposes of calibration - but my trusty old Chevy Lumina has always matched them down the mile.  (Of course, I don't actually know they don't fool with those readings....) Wnt (talk) 16:00, 30 November 2011 (UTC)
 * In California and in Kansas, those are often found just before school zones. 67.169.177.176 (talk) 03:17, 1 December 2011 (UTC)
 * They have those in NZ. I occasionally drove a car where the speedometer would show the speed as about 50 km/h but these signs (plural hence I was fairly sure it was the car not the signs) would show it as about 40 km/h i.e. an error significantly above 110% maximum of some countries mentioned in the article APL linked to. I don't believe the tyres were singificantly underinflated. However it was a fairly old car so I presume it just fell out of calibration. New Zealand does require a yearly or 6 monthly (depending on age of car) Warrant of Fitness where the speedometer is checked, I presume the tolerance allowed to pass the WOF is or was greater then 10% as I don't believe it caused a problem for the car. Nil Einne (talk) 03:24, 1 December 2011 (UTC)