Wikipedia:Reference desk/Archives/Science/2007 June 27

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June 27[edit]

Aphids among the thorns[edit]


On a trip to Peres river, I saw plentiful aphids on the thistly plants of the kind pictured here, but none on adjacent flora. A similar phenomenon has been observed at my window box with a different specie of aphids - there were about a trillion of them on the invasive Hedera helix, but none on nearby plants. What makes invasive plants so attractive, or detracts aphids off local vegetation? Thanks, Lior 02:10, 27 June 2007 (UTC)

It may be that the aphids prefer to hide amongst the spikes of the thistles so as to avoid predators such as birds that might not want to be pricked. SteveBaker 02:55, 27 June 2007 (UTC)
maybe as is the danger with any introduced species, since the local species evolved with the aphids they adapted to a balance with both species in check, where as the introduced species has no natural protection against the aphids and is therefore overwhelmed, a similar thing could possibly happen if that aphid was then introduced into that plant's native environment, but possibly the habit there is in other ways inhospitable to the aphid. Vespine 03:30, 27 June 2007 (UTC)
I hope other answers will be added, as I don't see how the Hedera helix case is explained by your interesting suggestions. There were some birds and lizards around, but I wouldn't count on them to spare the aphids among the thorns. As one can see, the aphids also covered easily reachable sections. Lior 20:03, 29 June 2007 (UTC)

Chief Executive Officers[edit]

What are the names of the past Chief Executive Officers of The Prince of Wales Hospital in Hong Kong?

This probably does not belong at the science reference desk. The official website has a history and a management structure chart. Wikipedia has a short article about this topic. Nimur 06:50, 27 June 2007 (UTC)

History of the "direct" current term[edit]

Would there be any science history enthusiast able to contribute somehow to the explanation of when did the term “direct current” appeared first in English, who proposed or introduced the term, and what actually the word “direct” suppose to imply in this context, meaning why and when did scientists started calling it “direct“ current instead of Galvanic current?

Was it because the graph is a straight line (cannot see any connotation), or because it is directly (!) from a battery (I doubt), or was it unfortunate mistranslation from Latin or other foreign language at the time, or is there some other explanation?

I am of electrical background and I know what direct current is, what I am after is the historical trace of the term “direct current” and its supposed meaning. I have spent many hours in libraries and on internet searching for answers, but with no luck at all. Many thanks in advance to anyone who can contribute or point in the right direction.

Did the term “alternating” current appeared about the same time?

You may be interested in the article War of Currents, probably answers most of your questions too. Vespine 05:21, 27 June 2007 (UTC)
At Google books if you specify full view you will be able to search books about electricity from the 19th century. I did not see a distinction of AC versus DC in the writings of Faraday in the 1840's or De La Rive from the 1850's. Once dynamos were invented and used for arc lighting, some were AC and some were DC. Early uses of the term "direct current {De La Rive, 1850's) were used to distinguish the battery current from the induced current, showing that the term did not then have the present sense. In 1884 I find the same Siemens dynamo described by one writer as a "constant current" machine and by another as a "direct current" machine. Further complicating things is that "constant current" often meant a DC generator with excitation such that the current stayed the same as the load was varied. I found the term "alternating current " popping up before the modern usage of "direct current." Both terms were probably known to physicists and electrical inventors in the 1870's. As Vespine said, the two terms were widely used by the war of the currents in the early 1880s. Similarly, 78rpm records were not called that until 33 1/3 rpm records were introduced; they were just called "records". DC electricity was just called "electricity" or "current electricity" to distinguish it from static electricity until AC gained some usage with transformers for sending current to distant locations. Edison 05:45, 27 June 2007 (UTC)
I'm on Edison's side here. I suspect that transformers created a distinction between direct current (a galvanic connection) and indirect current (through magnetic induction). But I have no reference to support my claim. Atlant 12:52, 27 June 2007 (UTC)
The Oxford English Dictionary (online subscription required) provides a quote from the Journal of the Society of Telegraph Engineers (XV p. 193): "I am glad that people are beginning to use the term ‘direct’ when they mean a current which does not alternate."; this quote dates to 1886, implying that the wide use of the term 'direct current' in the sense that we understand it today started to take hold sometime in that decade. (As others have noted, the first use of the term may well be quite a bit older.) The term was sufficiently established by 1889 to appear in Edward J. Houston's book A dictionary of electrical words, terms and phrases. TenOfAllTrades(talk) 15:20, 27 June 2007 (UTC)
Our sister project - the Wiktionary says: From Latin directus, past participle of dirigere ‘straighten, direct’, from di- + regere ‘make straight, rule’. - so it seems reasonable to use 'straight' as an antonym of 'alternating'. It also provides a meaning: 1. Straight, constant, without interruption. - also a reasonable use of the word. But I agree that it is almost certainly the other meaning: "directly from the battery - not indirectly through a dynamo" because early sources of AC would have come indirectly from some ultimate DC source. SteveBaker 16:29, 27 June 2007 (UTC)
Thank you guys, especially to Edison for the tip about Goggle books.

I have searched back to 1750, when Goggle gave up on me. Here is what I have found;

•Some US Government document published in1777 states: ….”direct and alternating current feeder wires shall be installed as follows:” …

•“The Discovery” published in 1763 refers to both the … “a.c. current from there to the receiver”… and further …”Low voltage alternating current is supplied”…

•“The Skipper” published 1753 provide interesting insight; …”alternating current generator, or alternator …’as is’ (my assumption)… presently called (meaning then in 1753)”….and further …. “The system is 12 volt and current is generated by what is termed a “three phase” Wico alternator.”

All this shifts the discussion a century back. It appears that both the direct and alternating current terms were well established in the mid 1700. Now, I hope there is still someone there who could dig deeper into the history and put further insight to this fascinating story? BrightSpark 09:21, 28 June 2007 (UTC)

A strong cautionary note here: Google books very frequently gets the date of a book wrong by hundreds of years, since apparently there is very little quality control of the people doing the data input. There is also the problem that when something is in "snippet view" you can't see the entire article to see the puiblication data. 1763 might be when "The Discovery's" first issue was published, and then Google shows you a sentence from something published 150 years later. This is shown by the supposed 1777 reference from "The Skipper" referring to "direct and alternating current feeder wires " because nothing like that existed in the world in that century, and the "US government" was more concerned with repelling the Redcoats than in a hypothetical electric power system for a hundred years or more later. As "history detectives" we may observe that it refers to the "volt" which was not a term in use until long after 1800 when Volta invented the battery. I would assign it 150 years or more later than the year you might infer from the snippet view. Another very poor feature of Google Books is that if you click onthe link to find the journal "The Skipper", apparently about sailing, at a library, it takes you to libraries that carry "Skipper" which is a different German magazine started in 2004 which is about lesbians. Likewise if you click on the link to find libraries with "Discovery: A Monthly Popular Journal of Knowledge" Google Books directs you to University of Arizona and a different journal called "Discovery." which was only puiblished from 1943-1966. All this is why I strongly encourage using only "full view" search for old public domain books to research 19th century technology. Edison 16:01, 28 June 2007 (UTC)
Thanks a lot Edison, after my last post I thought about all this and got to the same conclusion; something must be wrong, the language appears too modern. So, I agree fully with you, the conclusion was premature. BrightSpark 05:24, 29 June 2007 (UTC)

What is a 'return vector'?[edit]

Hi all. I was wondering what a 'return vector' is. I've heard the term in reference to 'coordinating points' (I'm not sure exactly what that means either!). It seems like a scientific term (I do know what a vector is), so I was hoping you could help me out. Much help appreciated ! Xhin Give Back Our Membership! 08:03, 27 June 2007 (UTC)

In what context did you hear about this? —Bromskloss 14:03, 27 June 2007 (UTC)
It may be the vector from Point B back to Point A (assuming that the vector from Point A to Point B is the original vector). Nimur 16:12, 27 June 2007 (UTC)
Hmm, so it could be seen as the vector that would provide a means of getting from somewhere back to wherever the source of the original vector was? Xhin Give Back Our Membership! 00:21, 28 June 2007 (UTC)
The direction you have to go to return to your starting position, perhaps. Hard to say without more context. Someguy1221 03:18, 28 June 2007 (UTC)

calling mobiles[edit]

hi, why is that calling a mobile from a landline so much more expensive than calling another landline? Also, whilst phone companies (at least in the UK) are falling over themselves to make calling landlines free, the cost of calling mobiles doesnt seem to move- any ideas? (also, my informants tell me that this isn't the case in the US - why?) thanks.. 10:47, 27 June 2007 (UTC)

Mobile phone networks are much more expensive to run than fixed line. Both need the same back-office equipment and trunk networking, but the mobile guys also need lots of cell towers and associated infrastructure, which have a per-user cost that's much higher than the cost of running a copper wire to your house. But moreover the mobile carriers paid crazy sums of money (in the UK, the US, and many other developed countries) to buy radio spectrum for 3G - an investment that they're struggling to pay back.
The cost differential between the US and the UK is a function of how cellphone billing is done. In the UK mobiles have their own area codes (077xx, etc.), while in the US cellphones are tied to regular area codes (212 for New York Cty, for example). If you call a mobile in the UK (something you can tell because of its area code) you pay more than calling a land line. In the US you can't tell you're calling a mobile, and the additional burden is paid by the mobile subscriber receiving the call. -- Synthetic element 13:04, 27 June 2007 (UTC)
"In the US you can't tell you're calling a mobile" because the FCC so decreed, for privacy reasons. New York City is an exception, having adopted 718 for mobiles before the rule was made (and before splitting the outer boroughs off from Manhattan). —Tamfang 22:03, 1 July 2007 (UTC)
As to why you can get free or nearly free landlines in the UK - really, you can't. You can get free or freeish phone service if you buy broadband and TV (a "triple play") from them, with the addition of mobile phone service too (a "quad play"). They do this because voice telephony is a commoditised, low-margin, cost-plus business (so there's not much to be gained from competing hard on it) while broadband, cableTV, and mobile telephony carry bigger profits and have much more opportunity for growth - so they compete on those, and chucking in telephony doesn't cost them much. For cable TV (Virgin) telephony is pretty cheap for them to provide - they own the last-mile cable and they own or lease lots of trunk network (mostly for the broadband service) so telephony is a drop in their bucket. Much the same is true for BT. Sky Broadband is really just a DSL service on a BT line, so they're having to buy the telephony function whole from someone else (either purely from BT or BT+someone's trunk). -- Synthetic element 15:21, 27 June 2007 (UTC)
Actually, according to one article I read, it's not true that "mobile phone networks are much more expensive to run than fixed line" or that they have "a per-user cost that's much higher than the cost of running a copper wire to your house". They can actually be substantially cheaper. In fact, according to this article, a lot of developing countries are skipping widespread deployment of land lines and leapfrogging straight to wireless.
Due to mass production of microelectronics, wireless phone handsets are cheap -- not much more expensive than conventional, wired telephones, these days. But stringing copper wires all over the landscape is fantastically expensive. Sure, cell phone towers cost money, too, but -- how many of them do you need?
If you're using wired lines, you need to string wires to everyone's house who wants to talk. And even if not everyone wants to sign up for phone service at first, you still have to run wires down the streets of everyone who might want to talk, because new subscribers won't be willing to pay for more than the drop from the street to their house. And this model only works for relatively densely-populated areas. Even in the U.S., it was close to 100 years after the invention of the telephone before everybody in rural areas had access to phone service, -- and there are still remote areas without it.
If the phone was invented in the late 1870's then you are saying rural folks in the US did without phones until the 1970's? I would say rural phone service was pretty universal by the 1940, and most small towns had phone lines going out into farm country by World War 1, so all but the most isolated areas had the service available if they chose to pay for it by World War 1. Farmers would put up the wires as a cooperative effort in some cases up like they were raising a barn. As for "everybody" I am sure some never got phone service at all (Amish and dedicated misanthropes and cheapskates). Today many younger folks have only celphones and never get a land line. Where are the "remote areas" without phone lines today? Thinly inhabited swamps, forests and deserts? Mountaintops? What county has no phone lines? Maybe in the third world countries this is true. Edison 13:31, 29 June 2007 (UTC)
Using a wireless network, on the other hand, if your towers have decent range, you only have to build as many of them to take care of the number of people who are talking on the phone at a given time. At first, when you have few subscribers, you won't need very many towers. (Or even if you have kind of a lot of subscribers, but they haven't gotten into the swing of things yet, where they're gabbing on the phone or messaging each other constantly.) As your subscriber base and usage grows, you can build more towers incrementally, and only where they're needed. So it's easier and cheaper to build a wireless network incrementally, as opposed to a wired network, where nobody can talk to anybody until you first sink the huge capital expense to crisscross every populated area with wires. —Steve Summit (talk) 23:53, 27 June 2007 (UTC)
Plus, when stringing a lot of copper wire through unpopulated areas, you have to deal with people who steal and resell the copper wire on the black market. -- JSBillings 14:24, 28 June 2007 (UTC)

Yeast Infection[edit]

For Candidiasis, does anyone know why or how the yeast overgrowth (or undergrowth) can be resistant to conventional over the counter treatments?

The same basic principles as all other instances of antimicrobial resistance apply. Profligate use of antifungals lead to selection of resistant strains, increasing the probability that the species causing any given infection is resistant to the most widely used treatments. If you were asking about specific mechanisms of resistance: [1] resistance to polyene antifungals involves changes in the ergosterol content of the fungal membrane; while [2] resistance to azole antifungals involve alteration of 14-demethylase, decreased intracellular drug accumulation, and loss of function of the enzyme 5,6-desaturase. - Nunh-huh 22:37, 27 June 2007 (UTC)

Mars Direct[edit]

I find Mars Direct to be a very efficent way of getting to Mars, but my only problem is the Methane Rocket technology involved in the ERV. Have methane rockets been tested or is the propulsion system still too young? 17:59, 27 June 2007 (UTC)

Apparently they have been tested. This [1] is recent, although the very first tests took place two years ago [2], it's even on wikipedia in the XCOR Aerospace article. Did you look in google? These were in the first page of search results on methane rocket. Donald Hosek 00:32, 28 June 2007 (UTC)

Why would spot or arc welding not electrocute someone holding non-insulated metal?[edit]

I've seen this a lot, usually on the Science Channel show How It's Made. A factory worker will hold two metal parts with ungloved hands while an automated spot welder fuses the parts. The weld is performed by the two copper alloy electrodes pressing against opposite sides of the part. I've also seen a robotic arc welder repair one end of a steel I-beam, while an operator standing at the other end steadies the beam, again with ungloved hands. Then there's the spot welding performed in a dentist's office. I've read that spot welding is typically low voltage while arc welding is much higher. But in any arc welding literature I've read, safety concerns listed are blindness and chemical toxicity, but not electrocution. You're passing current through metal, butt here's no risk of shock? So, what am I missing? Bonus question. If spot welding takes one volt, how come you can't hook two copper electrodes up to an AA battery, touch them to two intersecting metal strips, and form a weld? What's the difference? 22:36, 27 June 2007 (UTC)

An arc welder is a powerful device which can supply a huge amount of energy, often high amperage and fairly low voltage. but enough power to melt ferrous metal or to make a heavy piece of steel red hot. An AA battery has internal resistance which prevent very much current from being drawn from it. It might just be able to spot weld small pieces of metal (That said, I must say don't try it, because hot things can burn you or start fires). As to why the people don't get electricuted, I will leave that to a welding expert, but I would look at the areas of grounding, of how the current would divide between the metal pieces and the person touching it (that is draw the circuit diagram), and at the voltage which the machine is putting out. That said, there are many different types of welding equipment and there are probably setups where a person could in fact get shocked. Edison 23:07, 27 June 2007 (UTC)
When you're arc welding, the workpiece is grounded and the high-voltage arc is struck between the electrode and the workpiece. (Actually it's often not such a high voltage, after all, but that needn't concern us.) So the only way to get shocked would be to touch the electrode -- but you don't want to touch that, anyway.
There's lots of current flowing through the workpiece back to the place where the welding machine's ground electrode is connected, but that current would much rather flow through that nice, low-resistance metal than jump out and shock you.
Basically, you get shocked when some part of your body bridges between two spots that are at different voltages. Usually, one spot is at 0 volts ("ground"), and one spot is at some other voltage. Most of the world is grounded, and most much of the time, you are, too, so most of the time, touching anything that's at a voltage significantly different from 0 will give you a shock. But if you touch something that's at a voltage of 0, you don't get a shock, even if that something is carrying lots of electric current.
The story is slightly different for spot welders. They use very high currents (at low voltages), and the current is injected very close to the point that it's collected. Away from that spot, there's very little current flowing at all. (And at any rate, the voltage is too low to shock you anyway.)
Finally, as Edison already explained, an AA battery is simply incapable of supplying enough current/power/energy to do any spot welding. You need hundreds or thousands of amps to make a spot weld, while I think an AA battery is capable of supplying mere milliamps. —Steve Summit (talk) 23:23, 27 June 2007 (UTC)
A AA alkaline battery can put out several amps, and I think it might be able to cause a very thin piece of iron wire to weld to another.Don't try this, because the battery might explode. Edison 03:43, 28 June 2007 (UTC)
?? several amps?? (just asking)Gzuckier 17:40, 28 June 2007 (UTC)
I just tested a well used Eveready AAA alkaline and it put out 5.3 amps. I did not want to test a newer one or a larger battery because it might blow the expensive 10 amp meter fuse. Here the current is limited by the internal resistance of the AAA cell, the contact resistance (very low) the lead resistance, and the internal meter resistance. Do not underestimate little alkaline batteries in terms of how much current thay can deliver into a low resistance circuit for a few minutes. Don;t carry one in a pocket with keys and change unless you want to start a fire or have the battery explode (I have never personally known one to explode from high discharge rate but the manufacturers say it is a danger. Edison 13:22, 29 June 2007 (UTC)
I said "most of the time, you're grounded", but that was a little too strong. If you've wearing a pair of robber-soled shoes, and not touching anything, you're effectively insulated from ground, up to a potential of a couple of hundred volts. (That is, you could probably touch a 120V wire and not get shocked. But if you tried to touch a 1000V wire, the voltage would be high enough that it would probably punch through your shoes and electrocute you.)
This is significant because there are sort of two different ways to get electrocuted. One is to be grounded and to touch a live wire, but the other way is to be in contact with a live wire, and then touch something that's grounded.
If I were wearing a pair of rubber-soled shoes, and holding a bare, live, 120V wire in my hand, I would not get a shock. My body would be raised to a potential of 120 volts, but there would be no place for the current to flow. But then, if I reached out and touched a metal water pipe with my other hand, then I'd get a nasty (potentially fatal) shock. Or if you walked up, barefoot, and touched me, we'd both get a shock. (From my point of view, I'm in contact with a live wire but ungrounded until I touch you. From your point of view, you're grounded, and touching me is like touching a live wire. Now cue Psycho Killer by Talking Heads: "Don't touch me, I'm a real live wire.") —Steve Summit (talk) 23:35, 27 June 2007 (UTC)
Whoah, careful! You mean, I take it, a 120V direct current source. Many American houses have 120V 60-cycle alternating current; that'll give you a nasty shock even in your rubber boots. The reason is that your body has significant capacitance -- the current will flow into you from the wire and then back out into the wire, over and over again, and it doesn't need an outlet on the other side. --Trovatore 00:38, 28 June 2007 (UTC)
Hmm, how sure are you? (I do know about capacitance, and AC current may flow into and out of you and give you a nasty shock, but I can assure you -- true OR confession here -- it doesn't do that to me!) —Steve Summit (talk) 00:43, 28 June 2007 (UTC)
Well, I suppose I'm not terribly sure. I have gotten shocked; I don't really recall exactly what I was wearing on my feet. But I don't plan to experiment. --Trovatore 01:11, 28 June 2007 (UTC)
I turn off the circuit breaker before I touch a line-wire; even then, I only touch one wire at a time in case of residual capacitance or unexpected voltage. I think it's a bad idea to touch an live AC line even if you are wearing thick rubber boots. Nimur 01:21, 28 June 2007 (UTC)
If you want to work on dead electric wires safely, the most important thing is to make sure they're dead. This sounds like a tautology, but the sad truth is that many people are killed by touching wires that they only thought were dead.
Neon test lamp
Get yourself a neon test lamp (right), and use it religiously. Also, unless you're working in a single-family residence where you can be sure that you're the only one who'll be messing around with the fusebox, put a note right on the fuse/breaker. The last thing you want is for some enterprising person to wonder why the lights are out, and to find the right breaker and "helpfully" turn it back on, just when you're grabbing the wire. (Professional electricians in industrial settings don't just use notes for this purpose, they use padlocks.) —Steve Summit (talk) 11:36, 29 June 2007 (UTC)
Yeah, I got shocked by sticking my finger into a US 120V electrical outlet when I was 6 years old (on a dare, I'm not stupid or anything.. *ahem*) and I had thick rubber-soled shoes on. I assumed the shock was possible because my fingernail was touching the conductor while another part of my finger was touching the wall plate. Wow. I didn't expect so many answers on this. The welding-related ones do make sense. Sorta. I understand what you're saying, but the idea that 500A could be dumped into a girder, and I could hold barehanded onto that girder a few feet away and live just seems bizarre. It's like an electrolysis setup. Current flows all over, but some does get to each point. I'd figure at the least, there'd be enough current to give you a shock.
Think about it this way: instead of contemplating whether there's enough current to give you a shock, ask whether there's enough voltage.
Whether that steel girder is carrying 1 amp, or 10 or 100 or 1000, or 0 -- as long as its potential with respect to ground is zero volts, no current is going to flow from it to another grounded object, i.e. you. And if the resistance of the girder is low enough that the voltage drop induced by the current it's carrying is near 0, then if one point of the girder is at ground potential, all of it will be. (Contrariwise, consider a piece of metal that is elevated to a high voltage -- such as, for example, one of those wires strung on insulators on poles along the street -- and suppose further that, for whatever reason, it's not carrying any current at the moment. Does that mean it's safe to touch it, since there's "not enough current to give you a shock"?) —Steve Summit (talk) 04:49, 29 June 2007 (UTC) [augmented 14:03, 29 June 2007 (UTC)]
While we're on the subject, is there a law or rule that specifies what combinations of amperage/voltage are possible? If a static electricity shock can be thousands of volts, but has little amperage, is the opposite also true? That spot welding description would seem to say so. Can you really have a 1V source that, when touched, could dump 1kA and turn you to crispy human bacon? I posted a question about that before. Specifically, I can't fathom why two revisions of the same consumer product would have vastly different power requirements (12v 500mA vs. 6v 2000mA). 04:41, 28 June 2007 (UTC)
Yes Ohm's law: Voltage (V) = Current (I) * Resistance (R) or V = IR -Czmtzc 13:33, 28 June 2007 (UTC)

Any combination of current (amperage) and voltage is possible. The most obvious everyday examples:

  • Low current, low voltage: most electronics
  • Low current, high voltage: static shock
  • High current, low voltage: car starter
  • High current, high voltage: lightning

Starters draw up to about 1,000 amperes and are typically powered at 12 V. A higher voltage would be more convenient (lighter-weight wiring could be used), but the power source is a battery and that produces low voltage. In fact, 6 V was quite common a few decades ago. --Anonymous, June 28, 2007, 07:06 (UTC).

Well guys you are right and you are wrong. How could that be? OK, there are situations where for whatever critical reasons the maintenance or repair have to be done on live electrical equipment, and sometimes at much, much higher voltages than 120V. In these situations maintenance crew will use special (!) safety (!) equipment; rubber boots, rubber gloves and special mats and platforms, not to mention strict procedures and compulsory presence of a co-worker during the procedure. All this is exactly for the reason you mention, to isolate the person from the ground or earth potential so the current that develops through the serviceman body and the protective equipment is so insignificant that the person can work safely.

Where you go wrong is assuming that wearing any (!) rubber boots will always (!) protect you. At 120V, it may or may not, and the fact that it protected you once does not really mean it will maintain the protection the next time. Electrical safety equipment is made from special (!) rubber like materials, using special design and manufacturing processes, and for that reason it is usually quite expensive. The fact that something is made of rubber or plastic does not automatically mean it is electrically safe! Dielectric properties of such materials vary significantly, though they may look alike. On top of that, all electrical safety equipment is subjected to strict safety tests, is rated at specific maximum voltage it will provide the protection and is given an attest; you may see that electrician’s screwdrivers or cutters, or the multimeters are all marked accordingly, usually 1000V. Further more the equipment is subject to regular maintenance and tests regime during its service life to ensure that its protective ability did not deteriorate over the time or due to accidental damage.

The rubber boots you buy at you local hardware shop are not intended for that purpose. They may possibly protect you at 120V when new and dry but I would not dare to test their ability at any voltage. With time and wear the insulating properties will change significantly. The thickness of the sole will reduce, micro-cracks will develop and moisture from perspiration will accumulate; all this will reduce significantly its protective ability and suddenly the current will find its way to the earth exactly the same way as lightning finds its way from a cloud to the earth; you may be unpleasantly surprised if not harmed. So here is where you go wrong.

I too think, it is a bad idea to touch a live AC line even if you are wearing thick rubber boots, and I am saying this from my past experience. To ensure safety more than that is needed. It is not that I am in any way giving you support but if you ever consider, for whatever reason, to do such an exercise ensure that you have someone responsible with you at all times. Such a person must know where and how to isolate quickly the right circuit and what to do should the unthinkable happened, and without exposing any one to further danger. Live wires are dangerous and there are good reasons for all the warnings.

And, on the other issues mentioned earlier;

Generally, international standards quote the “Extra Low Voltage” limits as being 120V for DC and 50V for AC (mind you the 50V is RMS value). Voltages below and up to the above limits are generaly considered safe to touch under normal conditions!!! (You do not stand barefoot on a wet floor in the bathroom or in the rain; you do not have cuts to you fleshy tissue where you touch the wire, etc. under such abnormal conditions you can be assured you won’t be safe). These limits are summary of statistical tests and the limits may be slightly different in various standards or countries..

Typically human body under normal dry conditions have adequate resistance (primarily due to the dry skin which acts to our fleshy tissue like an insulation on a wire) so the current that develops through you body when you touch voltages below the above limit is so insignificant that it won’t affect you.

Safety standards further quote current limits for what is known as “Threshold of Perception” (lowest current which causes any sensation to a person through which it passes). These are 0.5mA for AC and 2 mA DC. Further more there is another yet limit called “Threshold of “let-go” (highest current at which a person holding electrodes can let go of the electrodes). These are about 10 mA for AC and approximately 300 mA for DC.

These figures are based on statistical data and are not absolute limits. Different people will respond differently under different circumstance, and there are many factors that affect this. So, one must be knowledgeable to interpret them. I have quoted them here to give you some understanding while you can safely touch the car battery or welding equipment, but should not touch the mains at 120V AC.

With the AA battery, yes it will give you a spark, but it will be so minute that you will be lucky to spot it in a dark room. It acts the same way as the arc welder. It simply does not have ability to maintain the current long enough; it wasn’t designed for that purpose. You will probably have more luck with a car battery. Remember however, such experiment is likely to damage the battery due to long term rapid discharge, especially repeated; again this battery was not intended to be subjected to such a torture. And one more, never ever leave a battery short circuited, even the AA. It will overheat due to excessive short circuit current and may explode!!!

And final one; the body capacitance is really of no consequence at 50 or 60 Hz and the theory expressed there appears to be confused.

Sorry, I thought I will do it in three sentences. As the subject is rather crucial it took me a while so, here we are. BrightSpark 09:06, 28 June 2007 (UTC)

Um, I wasn't confused, just (apparently) wrong. I hadn't done the actual calculations (and still haven't, but assuming the ones given here are correct then) that wasn't the reason I got the shock. But it could have been, in principle. --Trovatore 06:32, 29 June 2007 (UTC)
According to [3] the capacitance of the human body is 60 to 300 picofarads. Static electricity to as much as 15,000 volts can be stored in this capacitance just by walking across a carpet on a dry day. The electrostatic discharge when you then touch a grounded object or a metal pobject at a lower potential can amount to "tens of millijoules" of power, with a peak current of 7.5 amps and a peak power of kilowatts. An IEEE paper[4] gives 100-150 picofarad as the measured human body capacitance tested with an AC bridge and higher for static electricity, 200-400 picofarads. These amounts of capacitance, with 120 volt electricity at 60 Hertz would produce a very small amount of capacitive current, so the hazard would be conduction from the source of electricity through the body, to a grounded object or another conductor. Capacitive current would be (voltage)*2 π*frequency*capacitance. Now to calculate the current for 120 volts, 60 hertz and for 240 volts 50 hertz, since these are found in most residences of the world (remember this is capacitive current only, and the electrical conduction is a great hazard beside this).
  • 120 volts, 60 Hz, capacitive current for a 300 picofarad person= 14 microamperes
  • 240 volts, 50 Hertz, capacitive current for a 300 picofarad person= 23 microamperes.
  • (This is presented for discussion only and is not to be relied on for any safety related matters such as doing electrical wiring or experimentation. Do not try the experiment because any inadvertent contact with a grounded object or conductor could be fatal while in contact with something electrified). Edison 15:56, 28 June 2007 (UTC)
  1. I've been zapped when I tried to do TIG welding without wearing a glove. The voltage is somewhere below 110, so you live, but you remember to wear a glove.
  2. You can't get zapped from just one pole of the AC line, body capacitance or not. Both theoretically, and practically. In my stupid youth I had occasion to work on the electrical system of apartments where I didn't have access to the circuit breakers, involving occasional skin contact with one side of the line, and nothing happened.
  3. There are websites which describe how to weld with car batteries.
  4. There was a case years back where an electrical lineman was electrocuted, despite wearing insulating gloves. Turned out a nearly invisible pinhole in the glove was directly over his wedding ring, and when he grabbed the conductor with that glove so that the hole was proximate, that was it.
Gzuckier 17:40, 28 June 2007 (UTC)

You most certainly can get zapped from one pole of an AC line, if it is sufficiently high voltage. In high school I hooked up an induction coil (19th century device which has a step up transformer to change 6 V DC to several thousand volts AC with an internal interruptor), a 6 volt battery, and a swith, to build a spark radio transmitter. The problem was, when I turned it on the voltage would come arcing out from the light switch, which was only in the 6 volt DC primary circuit, and cause an excruciating shock. It was capacitive coupling from the ungrounded circuit. Likewise a utility high voltage line will arc impressively to an ungrounded helicopter when the workman repairing equipment on the line first makes contact. (They wear metal suits and have special training for live line work). Edison 13:11, 29 June 2007 (UTC)
Let me clarify, I don't want to try welding with a battery or car starter. That was just a hypothetical example given for comparison. BrightSpark's answer was very informative, but one point given confused me. Specifically, the section about the “Threshold of “let-go”. If the let-go threshold for DC is 300mA, does that mean if one were to clip the plug off a puny 3.5V 500mA AC power adapter adapter (or any standard wall wart), strip the wires and grab them, they wouldn't be able to release? Similarly, the anonymous post before BrightSpark said "Starters draw up to about 1,000 amperes and are typically powered at 12 V" Does that also mean that if one were to grab those leads, the car starter would instantly kill them?
I'll break in here to answer that point. If you broke open the starter circuit while the starter was in operation, you might get an impressive arc, but once the gap was opened wide enough, the arc would stop and then there would be zero current flowing. If you then "grabbed the leads" and put yourself in the place of the starter, which I think is what you mean, that would be a different circuit with a much large resistance in it, namely your body. The current that would now flow would be determined by the 12 volts and the resistance of the skin contact and your body in that configuration (plus the internal resistance of the battery, and the restistance of the wiring, but those are both too small to matter). In a worst case it might still be enough to hurt or kill you; this I can't say. But I can say it would be just about 1/10 of the amount of current you'd get if you grabbed two leads that were live with a 120 V household power supply, so the chance of it being lethal would be less. --Anonymous, June 28, 2007, edited 23:50 (UTC).
To the other two points:
  1. You can certainly weld with a car battery. A few years ago, some friends of mine built a go-kart powered by car batteries and starter motors. The fancy high-current solid-state motor controller they'd bought, the one whose specs said it could theoretically handle the currents involved, fried the very first time they used it. So they were reduced to using a little accelerator-pedal-like strip of metal which you pressed with your foot to complete the circuit. Trouble was, it tended to weld itself in place, so while you were zipping around in this thing, trying to not steer into trees or telephone poles or anything, you'd be madly trying to pry the "accelerator pedal" back up with the side of your foot...
  2. A "puny 3.5V 500mA AC power adapter adapter" can supply a maximum of 500 mA. It's not some magical device that shoves 500mA through anything it touches. (Though there are constant-current supplies which try to do this; more on them later.) —Steve Summit (talk) 04:43, 29 June 2007 (UTC)
I've seen conflicting posts that state the threshold for death is 2A, or around 60mA (I think. Could have been 600mA) across the heart. The former was on Wikipedia, and the latter was on the TV show Mythbusters. Anyway, the arc welding and rubber boots explanations make sense now, though it is a difficult concept for me to wrap my brain around. I do a lot of work with electronics, and always try to make safety a priority. But these simple parts of EE theory always manage to elude my understanding. For example, If I were designing a DC electrical device, and assuming the components could handle it, what would be the difference in supplying 6VDC @ 200mA or 12VDC @ 100mA? Isn't the output identical? As another example, what would happen if a consumer electronic device came with a 12VDC 1250 mA AC adapter, but it was swapped with, say, a 30V 500mA AC adapter? Isn't the output also identical? If not, that's what I just don't get; that there's more than one way to accomplish the same output, but those other ways fail. I know there's a way I can understand this. As I said before, I'm definitely not stupid. Hell, I once quit Mensa. :) 21:52, 28 June 2007 (UTC)
If you think that 12 volts at 1250 mA should be the same as 30V at 500mA, ask yourself this: if the recipe says to bake for 30 minutes in a 350 degree oven, does that mean you could also cook it for 350 minutes in a 30 degree oven? (This isn't an exact analogy, but it has the same, er, flavor.) —Steve Summit (talk) 01:10, 29 June 2007 (UTC)
The "3.5V 500mA" rating means that it will provide a nominal voltage of 3.5 volts, and a maximum of 500 milliamps. To determine the actual current, you divide the voltage by the resistance of the load across the terminals. Since intact human skin has a fairly high resistance, the actual current flowing will be under a milliamp.

If, on the other hand, you were to connect a one-ohm resistor across the terminals of the power supply, it would try to provide 3500 milliamps, overheat, and catch fire. --Carnildo 22:58, 28 June 2007 (UTC)

As I recall, the lethal current directly through the heart muscle is down in the tens or low hundreds of milliamps. And for this reason, pedants are find of saying, "It's not the voltage that kills you, it's the current." But this is a terribly misleading statement. The implication is that a 1 milliamp power supply wouldn't kill you, but that a 1 amp supply would. But you very rarely find true 1 milliamp or 1 amp power supplies. So what we need to understand is why a 12 volt battery almost certainly won't kill you, and a 120V wall socket might, and a 1000 volt power line almost certainly will.
Let's suppose that the resistance across your body is 1000 ohms. (I think that's about right, if you overcome the higher resistance of normally dry skin. Perhaps your hands are wet, or you're applying the electrodes to open wounds.) Suppose further that if you pass a current from one hand to the other, across your chest, one tenth of the current flows through your heart, and the other nine tenths flow through other parts of your chest. (This is a number I just made up; I have no idea what the real current distribution through your chest might be. But this is just a thought experiment; we're not going to try this or anything; we're just getting a rough feel for what the numbers might look like, to mix a metaphor.)
Suppose you touch the two posts of a 12V car battery, one with each hand. Ohm's law says that a current of 12V ÷ 1000Ω flows, or 12 milliamps. If one tenth flows through your heart, that's 1.2 milliamps. Not enough to kill you.
Suppose you put yourself across 120V. Now the current is 120V ÷ 1000Ω, or 120 milliamps. A tenth of that is 12 milliamps, and we're definitely in the danger zone.
Now suppose the voltage is 1000 volts, or one kilovolt. The total current is 1000V ÷ 1000Ω = 1 amp, and a tenth of that is 100 milliamps, and you're probably dead. —Steve Summit (talk) 04:43, 29 June 2007 (UTC)
P.S. A 500 milliamp power supply that's trying to source 3.5 amps will "overheat and catch fire" only if it's poorly made. Most power supplies limit their output current to their safe limit somehow (perhaps with a fuse), since short circuits are so easy to accidentally complete.
Sounds like you need to consider the difference between voltage and current. The total power might be the same, but a small amount of flow with a lot of force behind it is quite different from a large flow with very little force. Sometimes it's better to be shot by a BB gun vs caught in a car crusher, however, a water jet cutter can cut stone but Hoover Dam is just a giant stone holding up a lake. DMacks 22:38, 28 June 2007 (UTC)

There's one more point that's worth making, although we've drifted pretty far afield from the original question about arc and spot welders.

Some of you may be wondering, why is there this asymmetry between voltage and current when it comes to power supplies? Why does a five volt, 1 amp power supply always try to give exactly five volts, but is perfectly happy delivering any current less than 1 amp?

The answer is that we're talking about constant-voltage power supplies, which are by far the most popular kind. But there is such a thing as a constant-current power supply, and for those, the situation is exactly reversed. The supply will do whatever it takes to make sure that the desired current flows, regardless of the resistance of the load. The higher the resistance of the load, the higher the supply's output voltage goes. But constant-current supplies have a maximum voltage they're capable of delivering. If the output resistance goes so high that it would take more voltage than the rated maximum to induce the requested current, the supply ends up breaking its promise, and supplying less current than requested. —Steve Summit (talk) 04:58, 29 June 2007 (UTC)

Fascinating stuff. It's interesting how, despite the fact that electricity has been around so long, there's still so much debate with respect to its effects on bodies. On its face, the apparent simplicity of Ohm's Law belies the actual complexity of the subject. It seems like there's a different answer for every scenario. And let me tell ya, for those of us who tend to learn by thinking in absolutes, this stuff can drive you insane. I do understand the concept of nominal voltage/current from power adapters, as well as the concept of actual current passed in the event your body becomes part of a circuit. It makes sense when I think of it like this: Say I'm swimming in the ocean and a lightning bolt hits nearby. Despite the massive amount of power and decent conductivity of saline water, my chances of survival would be based upon how far away I was from the strike. The arc welder and girder scenario seems to be no different. If I understand the answers correctly, assuming the girder isn't grounded, any current traveling to the end of the girder (where the hand was) is overcome by the girder's resistance, gradually dissipates and is just converted to heat?
I guess my main problem is, as someone new to hobbyist-level electrical theory, all the variables make it difficult, if not overwhelming to reliably assess the danger potential for a given situation. For example, I do know that shorting a capacitor from a TV is a good way to make yourself quite dead, but for smaller, or less power-hungry electronics, (Ionic Breeze, clock radio, or an audio mixing board), finding the crossover from instant death/major pain/minor pain/no effect is a big gray area. Obviously, safety precautions should not be scaled down for perceived "safer" projects, but even with consistent safety, all these variables (moisture, temperature, proximity to, size, resistance & conductivity of the object, etc.) make it way more difficult than it probably should be. :)
One last question regarding differing voltage vs amperage values in power supplies. Would it be correct to think of a device's voltage requirement as being the average voltage required to complete the circuit (by overcoming the cumulative resistance of all the components) and amperage being the power required to bring all the components in that circuit to a functional power level? That's the best description I can think of that makes sense.
Anyway, thanks again for all the answers, they will definitely be kept and studied, probably repeatedly. :) 00:32, 30 June 2007 (UTC)