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Is that toothbrush on the internet of silly things?

Depending on the definition of "internet", this toothbrush might be. (It might not use the Internet Protocol running atop Bluetooth, but it does connect to a machine that probably is connected to the Internet.)

I like the idea of hacking someone's toothbrush.

That might be difficult if, as per the All About Circuits posting I referred to in a followup, there's a "code protection feature" in the microcontroller that makes it difficult if not impossible to overwrite the code. You might still be able to overflow a buffer and cause a return to code you supply, for example - if the microcontroller isn't a Harvard-architecture processor only capable of running code from code memory.

And, yes, there are people who want to hack their toothbrushes.

(For more fun, try a Web search for "toothbrush microcontroller" - that's how I found all those links.)

High voltage is only dangerous because it can enable large amounts of current to flow through your body (assuming large amounts of current are available), the current is what actually kills you. The IC that drives the display is only capable of driving 1 mA which is only barely at the threshold of perception and not in any way dangerous. http://www.allaboutcircuits.co...

http://www.bristolwatch.com/
http://www.circuitstoday.com/
http://forum.allaboutcircuits....
http://www.bowdenshobbycircuit...
And here's a neat project to make a lab power supply out of an old PC power supply http://www.electronics-tutoria...

"But why can't high-voltage lines be run underground?"
Don't you think someone would have done it by now if it were feasible?

Glad you asked. I'm trying to convince people not only is it feasible, it is high time to get on with it.

Air isn't a very good dielectric and in wet weather it gets even worse, see this list of insulator breakdown voltages. Glass has 40-100 times the dielectric strength of air, so yes, HVDC conduits ARE possible in standard sized trenches.

You have to realize that when most of the country was spanned, suspended cable on tall pylons in their wide right-of-way corridors was the cheapest and fastest way to do it. In many areas the real estate presently used for these, some of which is very valuable, can be reclaimed as it moved below ground.

Here is one company with a design for trench-able electric pipes that could handle 15 gigawatts at 800kv. That's 2.5 times Las Vegas summer peak load. No superconductors or refrigeration, just lots of aluminum. You'll also see a sad note at the bottom, "I have so far found that US-based venture capital investors will not take an interest in the elpipe because it is "too big, too long term."

This "too big, too long term" dismissal is symptom of serious problems. Venture capital investors, some who already have great-grandchildren, are refusing to even approach infrastructure repair and re-build projects in North America. What do they think the world will be like in 50 years if these things are not done?

Another company working on HVDC circuit breaker (check that photo, looks like fun). Also check out Roger W. Faulkner [2005]: Electric Pipelines for North American Power Grid Efficiency Security for some calculations on how much aluminum we're talking about.

Although you'll see a lot of talk about HVDC helping to make wind and solar 'renewables' more practical, I don't think so, because for base load power they are too expensive at any price.

Neither wind nor solar would save us from extinction in the case of a long harsh Winter or a climate disrupting global dust cloud event. On that point alone I believe every penny spent on big wind and big solar is wasted. I want my children to survive.

For the big picture on how I believe HVDC pipelines and reliable scalable base load power is the way to go, see

My letters on energy:
To The Honorable James M. Inhofe, United States Senate
To whom it may concern, Halliburton Corporate

I like the looks of your SdrDx app - how cool! I'm into SDR in a small way. I'm working on a TV dongle-based 6m propagation alert system. I currently have an RTLSDR-based dongle uploading ADS-B data to flightradar24.com.

I started in electronics just as tubes were being supplanted by solid state, so I know the basics, the variety, and some of the quirks but not enough to build anything tube-based running close to the edge of the envelope. I do recall someone I know running a HUGE 160m amplifier that had about 10kV on the plate. According to my physics prof at the time we were likely getting some soft X-rays off that thing. The guts of its single enormous tube glowed cherry red after a few seconds of CW. The 220V power supply had a variac with a knob the size and shape of the one you see on submarine doors, and the whole thing was the size of a 'fridge.

My favorite tube was the cats-eye modulation indicator.

I checked my log, but we haven't worked each other. Hope to see you on the bands! 73 de k4det

Which is why we have two systems: Conventional Current and Electron Flow. What you describe is electron flow.

Either can be used. Neither is superior to the other. Both work consistently, AS LONG AS YOU DON'T MIX THEM.

You will find conventional flow notation followed by most electrical engineers, and illustrated in most engineering textbooks. Electron flow is most often seen in introductory textbooks (this one included) and in the writings of professional scientists, especially solid-state physicists who are concerned with the actual motion of electrons in substances. These preferences are cultural, in the sense that certain groups of people have found it advantageous to envision electric current motion in certain ways.

http://www.allaboutcircuits.com/vol_1/chpt_1/7.htmlConventional current tends to be an electrical engineering convention. Electron flow current is a physics conventional preference. Unsurprisingly, partisans of one often complain the other is wrong. This is fanboi-ism, no less than Apple partisans complaining that Windows is wrong.

Not if he bought a fake.

Even if he bought a real one, the vast majority of them don't work very well. If you really want to prepare for the zombie apocalypse, stick with good old rechargeable AA or AAAs, a programmable charger, and either a generator or a large regulated solar panel.

low-quality transmitters emit more on side-bands, IIUC

"side bands" carry the information in the radio signal. They are created by the modulation of the carrier, and are what make the signal have "bandwidth". While a low quality transmitter may have some noise in the oscillator that appears as side-band information, it is probably not as much "in the side-bands" as a full power FCC licensed FM stereo radio station that has Muzak or other extended signals, also known as "SCA".

It is the poor filtering of the low-quality transmitter that results in the emission of harmonics (third, fifth, etc.) from a non-linearity in the oscillator or the amplifiers. In this case, a third harmonic around 312 MHz, which is a common unlicensed control device frequency.

http://forum.allaboutcircuits.com/showthread.php?t=15170

The line filtering and surge suppression in a UPS is active all the time but the batteries are not. A ferroresonant transformer will smooth out transitions in both directions; it is an always on device.

http://www.allaboutcircuits.com/vol_2/chpt_9/6.html

The transformer does smooth fast transients from either load or line but there is still variation. I've only used the ones for computers that have a clean sinewave out.

There are ones so noisy they'd swamp any line monitoring but using them for electronics is not a good idea.

I found this more informative on the reluctance motor http://www.allaboutcircuits.com/vol_2/chpt_13/4.html. Of course you would still want to incorporate solar panels on the top surfaces of the wings just to get that bit of extra free energy into the system, especially if done properly as the wing surface so no additional weight. Add to that an inflatable aircraft http://bayourenaissanceman.blogspot.com/2008/06/weekend-wings-20-inflatable-aircraft.html (using hydrogen or helium) and away you go.

Nope.

Electric wall clocks that you plug in use the AC line for accuracy.

The 60Hz out of the wall socket is very accurate. Accurate to within a minute or so a month. They use something called a synchronous motor. It's only in the past 40 years that quartz crystal controlled clocks were even mass marketed.

"Single phase synchronous motors are available in small sizes for applications requiring precise timing such as time keeping, (clocks) and tape players. Though battery powered quartz regulated clocks are widely available, the AC line operated variety has better long term accuracy-- over a period of months. This is due to power plant operators purposely maintaining the long term accuracy of the frequency of the AC distribution system. If it falls behind by a few cycles, they will make up the lost cycles of AC so that clocks lose no time."

http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

Even plug-in alarm clocks don't use a crystal oscillator - they simply count pulses from the AC line.

When Southern California Edison went from 50 to 60Hz in 1948, people had to throw out their old electric wall clocks and get new ones.

--
BMO

In the US we have single split-phase power delivered to most homes. We have a transformer that converts distribution voltage (19,900V in my neighborhood) to a 120V/240V secondary. We can either have 120V or 240V circuits, depending on the load. The power is delivered to a breaker box where the breaker positions alternate which phase they're on. That way two adjacent 120V breakers are on different phases (for load balancing), but a double-side 240V breaker uses both legs.

See also http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

Depends on where you live. In the USA and Canada, black is hot.

Of course it will immediately be adopted across police departments because as we all know tasers are perfectly safe. It is interesting to note when officers fire their pistols, they continue firing until the ammo is depleted. There is no reason to believe this practice won't continue with semi-automatic taser guns because many taser deaths were due to multiple hits from several officers. Of course these occurred because the suspect would not stop flailing about on the ground due to being repeatedly hit with electricity (officers refer to this as resisting). That is merely the unfortunate side effect of electricity causing involuntary muscle contractions.

http://www.youtube.com/watch?v=OuXR0F6ZQzc

That's a great list, I have every one of those and use them often, but it has a very steep starting point.

Books like Gettng Started in Electronics by Forrest Mimms, Practical Electronics for Inventors, Tab Electronics Guide to Understanding Electricity and Electronics (2000) by Randy Slone, Teach Yourself Electricity and Electronics (5th ed.) by Stan Gibilisco, Grob's Basic Electronics by Mitchel E. Schultz, or MAKE: Electronics: Learning Through Discovery by Charles Platt are more suitable first book for a starting point.

Also ARRL's Ward Silver has a great little hands-on book of lessons, ARRL's Hands-On Radio Experiments that is cheap ($20 US) and a great 2nd book. (Electrical Engineering 101 2nd. ed by Darren Ashby is another great 2nd book, oriented to new EE students / grads).

Make-zine and their blog are full of interesting hobbyist oriented stuff for beginners, and cool projects to inspire you to learn more.

For licensed amateur radio operators, the QRP community and their own QRPedia is a area of kit-building and home-made of simple radio transmitters and receivers that can be simple to get started, and fun to operate (as the solar cycle improves).

All About Circuits is partial (unfinished) online basic electricity and electronics textbook.

Fun stores (of many) include Ada Fruit Industries, and SparkFun.

The more hobbyist friendly big parts distributors in US are Jameco, Digikey, and Mouser. Anyone interested should request a catalog from them. They also ship to Canada, and Digikey does operate in Europe, but Farnell is generally better to deal with. G3SEK's Technical Topics website includes a list of UK electronics suppliers that deal with individuals (rather than businesses-only).

Enjoy!

Try: http://www.allaboutcircuits.com/ This has some good lessons describing the prinicples behind circuitry, and suggests some experiments to try. Best of all, it's free!

I used this site throughout my engineering degree. http://www.allaboutcircuits.com/ Covers everything from a basic intro all the way to AC, DC, op. amps, transformers, etc... all nicely formatted.

http://www.allaboutcircuits.com/ Very good and free info.

You're right. This is what I love about slashdot. I see that electronics technicians are sometimes too cocky and think they know everything. However, I always thought the two hot legs from a center tapped transformer were 180 degrees apart and therefore two separate "phases".
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html near the bottom of the page.

Oh, yeah--no FM either.

Say what? Please try one search before posting nonsense. Thanks.

Depending on the circumstances 5 watts is more than enough to kill someone.

I thought we were talking about power

For a given amount of power, if you double the voltage (and thus halve the current), you halve the resistance. OR, you reduce the copper and keep the resistance the same. Or any combination thereof.

Higher voltages = less loss and less copper.

I must have missed something during my electric classes. Here's what I remember:

P = VI = RI^2

The reason why P remains the same while V and I compensate each other should be precisely because R is supposed to be constant. So I still don't see the huge variation you mention. Maybe you could still point me to that scientific reference explaining it.

I never expected a 240V equipment to care about neutral if it had the expected ground (for safety), but that doesn't seem to be the case, although I have no clue as why

Because if you draw 120V on a neutral-grounded 10-30, you dump current to ground.

But that's the thing. Forget the ground for a second. There should not be a need for a neutral on a 240V outlet if the device is going to use 240V. It doesn't matter if one is neutral while the other carries all the 240V, or if they are both hot and opposed by 180 degrees.

If you want to take 120V from there, you should be on a 120V outlet.

We all agree that the ground is not for feeding the device. It's for keeping its metallic parts free of current, for safety.

But if we do use the ground, a neutral-grounded 120V grid works just as well. There's no power going to the ground. It just becomes the reference.

There are safety problems I can imagine with that, tho:

1. Wrong wiring or "smart" users: if the device relies on neutral to be grounded but the outlet is badly wired or the user manages to reverse the plug, there will be 120V waiting for him on the metallic grounded parts
2. Wire resistance: the little resistance of the ground wire could make the device's end of it to show some volts instead of being truly grounded
3. Lightning strikes: the ground close to where the lightning stroke may not be quite the same as in other places and that can cause a surge on devices if their neutral is shorted to the ground.

Our body's resistance is roughly the same regardless of the voltage applied, so we'd get a worse kick from 240V than from 120V.

True, but your common sockets would be rated for half the current. No need to up the average power of the average plug. Double voltage, half current. And either half resistance or less copper.

I get the impression that you fell on the same fallacy as another comment regarding how much current an outlet gives.

When it says 120V/20A or 240V/10A it doesn't mean that it carries that many amperes all the time. It does carry that many volts, but the amperes are directly proportional to the resistance applied. Our body.

Here's a very interesting article about that.

Have you considered improving your ground connection? This will considerably reduce your shock hazard.

For those of us not in the know, can you explain which parts of what is in essence little more than two large coils of wire can be salvaged when the wire has caught light?

The one with the less burn marks on it Mr. Smartypants. :P

Exactly because transformers are little more than coils of wire submerged (or not - some, smaller ones, rely on air-cooling) in oil-coolant - there is not that much to go wrong when it does go wrong.
Unless they fuse into a ball of copper you can even rewind the coils and build transformers from two completely useless ones - just as you can build a small transformer by hand.
Sure, you can't do that at home with a Leatherman and a stick of gum - but you CAN do it in a specialized machine shop.
The kind that can be found at the premises of power companies.

Also, here is a textbook example of a transformer.
Note multiple coils and separate coolant tank.

Are you telling me that out of perhaps.. 20... of such devices ALL of them will be so ruined that no single coil or a liter of coolant will be salvageable?
In a case of such an event I am guessing that we have nothing to worry about - electromagnetic radiation will fry us anyway.

How? We're not just talking about supply problems, we're talking distribution as well. With powerlines and substations down, there's no way to switch who gets the power.

Ah! I'm glad you asked that.
There is this set of secret skills they teach all power-technicians.

Like WALKING and FLIPPING SWITCHES BY HAND! Or even DRIVING to the grid-junction.
In a case of an emergency they can even coordinate their work down to a minute - by LOOKING AT A WATCH!

Again, the scenario in TFA is about complete blackout, not intermittent supply -- this is on a completely different scale.

The 4-hours-a-day scenario I was talking about is not the start of the situation.
That is the second or third day scenario, when SOME power becomes available and distributable.
THAT can last for days, weeks, months - getting better incrementally. Fixing things city block by city block.
4 hours turn to 6, 8, 12... Then you get 24-hour power with short breaks here and there. Then... things get back to normal.

http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

That link has good information for anyone interested in the actual math. The short of it is that if you grabbed the terminals with metal pliers, your resistance would be 2500 Ohms at worst. 12 volts divided by 2500 Ohms is only 4.8mA if my math is right, far short of the 500mA lethal dose.

If you were doing this just after getting out of the shower, your resistance would be 500 Ohms which would equal 24mA (again, if my math is right) which is still far short of the 500 needed to kill you.

I agree that the NEETS modules are a great resource, but seriously anyone who has ever tried to read them can tell you that you will fall asleep after chapter 1. Trust me, don't do it unless you have to. My suggestion is to start in small doses, NOT using the NEETS, use a site like: http://www.allaboutcircuits.com/vol_1/index.html to get the basics down, I still use it as a refresher, and then if you feel you are ready for a more in-depth reading on the topic look in the associated NEETS module for the full-monty. Good luck!

I agree completely with parent. A few additional thoughts:

Although I second the suggestion to get a book, I'd also suggest the following website: All About Circuits. It's basically a short textbook, online. It has some nice intuitive explanations.

As for books... My top choice would be Hambley's Electronics. It's a complete, correct, and accessible introduction to the subject. It's a great book. The Art of Electronics is also very good.

I also completely agree with the suggestion to get a solderless breadboard. That's the way to have fun with this stuff. You can always build a soldered, "final" circuit later (which is fun in its own way), but I have to admit that that's more of an exercise in fabrication than it is a good way to explore electronics.

For me, the crucial central component of a lab bench is an oscilloscope, and that will be the hardest thing to get inexpensively. Digital scopes are wonderful! I haven't investigated this thoroughly, but you might go with a USB "oscilloscope" that uses a laptop/PC for its interface, as these tend to be cheaper.

Finally, there's the question of "what circuits should I build?" Personally, I always found op-amp circuits to be a lot of fun, and I think audio circuits are often a good choice as they are interesting and practical, give a good way to experiment with filters and many other analog signal processing circuits, and yet are low-enough frequency that the parasitics are negligible (i.e., the schematic is an accurate representation of what you've built).

As a side note, although the 741 is an armored tank and as cheap as dirt, my personal favorite op-amp is the LM6132. They're more expensive, but man are they beautiful! :-) (Seriously though, just buy 741s unless you're running off batteries or really need something faster.)

Oh! And while we're on the subject of buying things: The place to look is Digikey. You need to lump together orders to save on shipping, but it is almost always the best choice for buying chips.

If you are into (or want to be) audio

http://tangentsoft.net/audio/

Else

http://www.allaboutcircuits.com/

+1 on EPfM.

That book is very light on theory, though. But it will get you building things. A good site for very basic electronic theory is http://allaboutcircuits.com./

Parent discusses music related electronics. The original question didn't mention music related anything, but if by chance you ARE interested...http://funwithtubes.net/ is great. Just finished building my first vacuum tube guitar amp, and it sounds GREAT. I had no idea how to do this stuff, just did a lot of reading and learning. The two sites I linked above were tremendous resources. And I got more, so if anyone is interested I'll provide them.

Hi,

During my computer engineering degree I often made use of this website as an additional resource:

http://www.allaboutcircuits.com/

It provides a great fundamental basis for all electronics and electrical circuits from simple DC/AC all the way to *FETs and OP-Amps. Enjoy!

http://www.allaboutcircuits.com/ Great site, use it all the time as a reference. Also I suggest, if you are actually serious about learning electronics, visiting several electronics based forums and just reading up on the latest posts. You pick up random information that helps build up the diversity of your knowledge. http://forum.allaboutcircuits.com/ http://www.electro-tech-online.com/ http://www.dutchforce.com/~eforum/index.php There's more, but that is the main ones for me (not including AVRFreaks hehe).

http://www.allaboutcircuits.com/ Great site, use it all the time as a reference. Also I suggest, if you are actually serious about learning electronics, visiting several electronics based forums and just reading up on the latest posts. You pick up random information that helps build up the diversity of your knowledge. http://forum.allaboutcircuits.com/ http://www.electro-tech-online.com/ http://www.dutchforce.com/~eforum/index.php There's more, but that is the main ones for me (not including AVRFreaks hehe).

http://www.allaboutcircuits.com/

It's still a work in progress, but it's mostly done by now and really well-written as an introductory guide.

Um, how exactly are you supposed to clean a low pass filter?

Or is this use of "low pass filter" something else?

vaporware. He promised a four-layer diode:

http://www.allaboutcircuits.com/vol_3/chpt_7/3.htm l

and never delievered on it. Some samples were made, but it never made it to production.

I've just completed a electrical engineering degree, and I can't stress how important the fundamentals are. This online book got me through the first 2 years of my degree:

http://www.allaboutcircuits.com/

Knowing exactly what each component does, and what effect it has when combined with other components is paramount to understanding more complex circuitry. Even in a digital system there is a chuck of analogue stuff which will really confuse you if you don't know what it does (and whether it affects the operation of the digital part).

Apart from the basics, the rest is all exposure really. Finding out how some effect is created, what techniques they've used, why they work the way they do, you'll learn all of this as you are exposed to more and more circuitry. You will definitely get overwhelmed if you try and learn everything though. For basic exposure an undergrad introduction textbook will serve you fine, once through that find out what your really interested in and read the recommended textbooks from any university.

Just fyi, in my electrical degree there are about 5 streams which you can specialise in, each which take about 2 years of concentrated study. And that's just at undergraduate level! (I'm in australia and our university system is different to that of the US, an undergrad ee degree is 4 years straight out of high school)

Lots of suppliers and circuits
http://www.makezine.com/

Lots of suppliers and circuits
http://www.nutsvolts.com/

Online textbook
http://www.allaboutcircuits.com/

Forum
http://forum.allaboutcircuits.com/index.php

Lots of suppliers and circuits
http://www.makezine.com/

Lots of suppliers and circuits
http://www.nutsvolts.com/

Online textbook
http://www.allaboutcircuits.com/

Forum
http://forum.allaboutcircuits.com/index.php

Instrumentation amplifiers. They're cheap and astounding. CMRR's of 80 or 100. Take a look at the Analog Designs AD620, for instance. It's superb (even if they are our competitors.) I've used it for making an EKG based on an old Scientific American Amateur Scientist article, and here's a slashdot thread about another AD620-based EKG.

I recommend this website especially the section on Health and Safety before someone gets killed from following electrical safety advice from Slashdot. Some really good advice about lockouts, measuring supposedly dead points 3 times (once to see if its live, once against a known source, and once to make sure your meter wasn't faulty the first time) and making first contact using the back of your hand.

No, heat is not a source of energy - it is the wasted part of energy we've used. (otherwise why do we have airconditioners?)

Many years ago I designed a city that filled a 3km cube. The limiting factor by numerous orders of magnitude was waste heat. The entire outside ended up being an airconditioner dumping heat.

David Holmgren (inventor of Permaculture) points out that maximum power transfer (http://www.allaboutcircuits.com/vol_1/chpt_10/11. html) applies to any energy collection system. Aiming for 100% is as bad as aiming for 0%.

But probably not at 48V. There is an excellent article which among other things discusses the relative risks of AC and DC. The problem with AC is primarily fibrillation.

In the US, a 10kW water heater would use exactly the same size wire as an electric stove.

Electric water heaters are used in a great many homes in the US and Canada. A typical residential American electic water heater holds between 30 and 50 gallons and is connected to either a 240V 30A or 240V 40A circuit. 120V residential water heaters are extremely rare. A 10kW tankless water heater in the US or Canada would be provided with it's own 240V 50A circuit, the same as a kitchen range would use.

Note that almost all homes in the US and Canada have split phase electrical service with two hot wires and a neutral wire (neutral is usually at earth/ground potential). In the US and Canada most wall outlets are ~117V and most large residential electrical appliances are supplied with ~240V. See http://www.allaboutcircuits.com/vol_2/chpt_10/1.ht ml or http://en.wikipedia.org/wiki/Split_phase

I meant to say energy. If I'm not talking to another electrical engineer I tend to slip into layman's terminology.

Then there are the people who say "kilowatts per hour" and arrogantly assert that they actually know something; they're either trolls or ignoramuses and I'm heartily sick of them. It's good to prove that you aren't one of them at the outset.

A battery with a capacity of 60KWH is not neccessarily fully charged if you run 200A @ 300V for an hour, as a significant portion of the incoming energy is dissipated as waste heat.

So, do you have any figures as to the charging efficiency of said batteries?

• 70% coulomb efficiency for NiCd, nearly 100% for Li-ion.
• Another claim of 100% coulomb efficiency for Li-ion, and 85-90% overall efficiency.

I understand that lead-acid efficiency is particularly poor because of the need to overcharge them to prevent sulfation, but my cursory search found nothing on that. Familiarity with their web site did let me find the efficiency graph on page 41 of this paper, but I doubt that a search engine would have. Those figures are interesting, showing small-cycle efficiency no lower than 90% over the entire charge range even for lead-acid batteries.

Your professor is a bit confused about the electric chair business: it was Thomas Edison who claimed publicly that Westinghouse's 60 Hz. system was much more dangerous than Edison's own direct current power system. This was strictly a marketing ploy: he and Westinghouse were going head-to-head in an all out corporate war and Edison wanted to win, badly. He had no scientific basis for his claims. In other words, he lied, publicly and repeatedly.

No that's not true. Edison had plenty of data from animal research. He even had a traveling road show that demonstrated (on sheep mostly) that they died when a comparatively much lover AC voltage was applied than when a DC voltage was applied. "You could turn the dial much higher." And it wasn't all sheep, they even electrocuted an elephant in New York (that had been condemed to death for killing its keeper). He even had billboards that said: "Don't use the executioners electricity in your homes!" (or words to that effect).

AC was indeed used for the first electrocution, suggested by Edison who build the apparatus. Westinghouse wisely refused to sell any equipment but then Edison arranged a purchase in secret and shipped it in unmarked crates to the place of execution. The first electrocution itself was a horrible botched affair, where many of the witnesses fainted from the stench of burning flesh. And the condemed man was first thought to be dead and the steam let out of the engine, only for the officials realising that he was still alive and everyone having to wait for the steam engine to be fire up again.

Also, your statement that DC is more dangerous than AC is not quite as straightforward as you make it to be. While at higher currents DC does tend to lock the skelettal muscular system more readily than AC; that doesn't in fact kill you as easily as electricity induced teatanus of the heart tends to resolve once the current is removed (hence defibrilation units use of DC), i.e. the heart starts again. AC otoh tends to cause fibrilation of the heart, which won't resolve itself and kill the patient (unless defibliration is available). We're speaking here of 50/60 Hz AC of course, as AC in the ten-kilohertz range or so is practically safe due to skin effects (your skin is a pretty decent conductor as other's have pointed out). Also, AC will also induce tetanus, though I'm not sure about 120V in the common case (Europe being on a 400/230V system. 230V can be enough to 'stick you on the circuit'.)

The best links I could find was this and this. Note the table half way down on the second page that lists the amperage needed for various effects on the body. I've had a better link before, but I can't find it now.

Your professor is a bit confused about the electric chair business: it was Thomas Edison who claimed publicly that Westinghouse's 60 Hz. system was much more dangerous than Edison's own direct current power system. This was strictly a marketing ploy: he and Westinghouse were going head-to-head in an all out corporate war and Edison wanted to win, badly. He had no scientific basis for his claims. In other words, he lied, publicly and repeatedly.

No that's not true. Edison had plenty of data from animal research. He even had a traveling road show that demonstrated (on sheep mostly) that they died when a comparatively much lover AC voltage was applied than when a DC voltage was applied. "You could turn the dial much higher." And it wasn't all sheep, they even electrocuted an elephant in New York (that had been condemed to death for killing its keeper). He even had billboards that said: "Don't use the executioners electricity in your homes!" (or words to that effect).

AC was indeed used for the first electrocution, suggested by Edison who build the apparatus. Westinghouse wisely refused to sell any equipment but then Edison arranged a purchase in secret and shipped it in unmarked crates to the place of execution. The first electrocution itself was a horrible botched affair, where many of the witnesses fainted from the stench of burning flesh. And the condemed man was first thought to be dead and the steam let out of the engine, only for the officials realising that he was still alive and everyone having to wait for the steam engine to be fire up again.

Also, your statement that DC is more dangerous than AC is not quite as straightforward as you make it to be. While at higher currents DC does tend to lock the skelettal muscular system more readily than AC; that doesn't in fact kill you as easily as electricity induced teatanus of the heart tends to resolve once the current is removed (hence defibrilation units use of DC), i.e. the heart starts again. AC otoh tends to cause fibrilation of the heart, which won't resolve itself and kill the patient (unless defibliration is available). We're speaking here of 50/60 Hz AC of course, as AC in the ten-kilohertz range or so is practically safe due to skin effects (your skin is a pretty decent conductor as other's have pointed out). Also, AC will also induce tetanus, though I'm not sure about 120V in the common case (Europe being on a 400/230V system. 230V can be enough to 'stick you on the circuit'.)

The best links I could find was this and this. Note the table half way down on the second page that lists the amperage needed for various effects on the body. I've had a better link before, but I can't find it now.

Ohm's law really only applies to linear circuits and linear elements (e.g. conductors, resistors). It doesn't apply to insulators (i.e. air) or semiconductors, which have non-linear V/I behaviour.

For an insulator such as air, you must look at its dielectric strength as well as the distance. From this, you can calculate the breakdown voltage required to generate an arc. For air, I believe it's roughly 10,000 V / inch. Of course, this depends on factors such as humidity, temperature, pressure, gas composition, etc.

If you apply a voltage below the breakdown voltage, no current flows. If you exceed the required breakdown voltage, then you get an arc. Essentially, this is a path of ionized gas, which is a very good conductor. This path is maintained as long as the voltage difference exceeds a minimum threshold (lower than the initial breakdown voltage).

For more info about nonlinear conduction: http://www.allaboutcircuits.com/vol_1/chpt_2/5.htm l