# Dan's Data letters #146

(page 2)

Publication date: 29-May-2005.

### Lighting yo'self up, continued

I read your electrocution article and I looked a few things up, and got curious.

Scientifically and mathematically, everything makes sense about the 9 volt battery Darwin Award, but why is it that when one licks a 9volt battery it is only a slight electric current (maybe 1mA, I imagine)?

I know liquidity lowers ohms, reducing resistance, but if the tongue is considered an external part of the body then despite the wet factor it shouldn't produce a shock. If it's considered an internal part of the body, shouldn't it be fatal?

Also, if you were to take a 9 volt battery and put two probes in buckets of water and stick your hands in the water, it should hurt you pretty bad (this PDF says hands submerged in water have resistance of 100-300 ohms, creating a range from 30mA to 90mA).

I'm a little afraid to try this, but I'm almost certain that nothing at all would happen. I tried wetting my hand and holding two probes because that should give me a minimum of 1.5mA (9/6000), which I should feel, but I didn't.

If injury was the case with the water buckets, you would hear more stories about someone walking in the rain with a 9 volt battery who got fried.

Kevin

The resistance a 9V battery will see across your tongue depends on how hard you press it on there and the exact chemistry of your saliva (if you just ate something salty, your saliva will be more conductive), but if my tongue's representative, the fiddling I just did with a multimeter suggests that around 70 kiloohms is a reasonable figure.

This is a simple DC circuit, so we can use V=IR (voltage in volts equals current in amps times resistance in ohms) to figure out what current flows:

9=I*70,000

so I equals 0.13 milliamps. Even if you had unusually conductive saliva, and pressed good and hard, and got the resistance all the way down to 20,000 ohms, you'd still only be talking 0.45 milliamps. This is why a 9V battery to the tongue tingles, but doesn't hurt; you'd need at least a couple of milliamps for real discomfort (been there, done that).

The reason why the resistance is still way up in the thousands of ohms is that the tongue, wet though it is, still has a skin-like surface, composed of the taste buds and their supporting structures and so on. If your tongue were just a bleeding piece of meat then the resistance would be much lower (flesh and blood is salty, and quite conductive); for the same reason, stabbing a couple of probes in there and hooking them up to 9V would result in much more current flow (and much more pain).

Even then, though, you wouldn't actually be in danger, because the current would just be flowing from one contact point on the tongue to the other contact point - not through any essential systems of your body. If, say, 50mA flowed through your tongue (quite enough to stop your heart) then it'd definitely be agonising; sufficient current through the tongue could cause permanent nerve damage (or just cook your flesh, in an extreme case). But even then, you wouldn't die, because the current would have no reason to go anywhere near your heart, your brain, or anything else essential to your survival.

Regarding the immersed-hands thing - a bit of experimentation with a multimeter, the kitchen sink and a saucepan has just showed me that I can get the resistance across my body down to about 5,000 ohms, with both hands immersed in clean water in metal-walled containers, and meter probes connected to those containers (which're otherwise insulated from each other, of course; if I put the metal saucepan straight on the metal sink-top without putting the breadboard under it that I did, there'd be an all-metal path from one probe contact to the other, and my body would have functionally no effect on the circuit no matter what I did).

Interestingly, the resistance I got in this experiment slowly rose, presumably because the cold water was causing blood circulation in my hands to fall.

Adding some salt to the water got the resistance down to about 3500 ohms.

Across that resistance, 12 volts would cause about 3.4mA to flow, which might be noticeable or might not; it certainly wouldn't hurt you. About a hundred volts would give a pretty sure 30mA across the chest, but even that wouldn't necessarily kill you; 30mA across the heart is a quite definite killer, but there's more to a chest than a heart, and the current would be spread out somewhat.

(Opinions differ regarding exactly how much current across the heart is likely to be lethal. It depends very much on how long the current flows. Apparently only about 6mA can put you into fibrillation if it's kept up for a few seconds.)

Why didn't I see the 100-300 ohms (times two, for two hands) mentioned in the PDF you mentioned?

Well, for a start, the hand-to-hand resistance of a human body under the skin is in the vicinity of 500 ohms - your left hand isn't connected directly to your right hand, after all.

Frankly, I think even the upper 300 ohm figure for immersed hands by themselves is erring on the side of caution, for people whose hand skin is intact. If you've been injured, though (as people often are, in electrocution situations - if nothing else, high voltage at moderate current can blast the skin clean off, leaving fresh meat that provides a much better contact for more current to flow), then low-ish voltages in a situation like this could be dangerous. Hand-to-hand current flow goes quite neatly across the heart; if you're going to die by electrocution, that's probably how it's going to happen.

In the real world, though, it should be noted that people work every day with 24 and 48 volt power systems on trucks, boats and so on, in wet, salty and filthy conditions, and get cuts and grazes, and the electricity doesn't kill them. They might wedge their watch band between a chassis rail and the positive terminal of a 24V 1500-cranking-amp truck battery and get a horrible burn; they may drop a spanner across the terminals and watch it weld itself in place and start glowing (that's a good time to start running). But they probably won't get zapped.

A large proportion (probably the majority) of higher voltage electric shock injuries aren't directly from the electricity, either. They occur when someone gets a quite un-dangerous jolt that causes them to fall off a ladder, fall down and whack their head on something hard, lose control of a power tool, stab themselves with a screwdriver, or do something even more amusing.

Household mains power should still be treated with great respect and unquestionably does kill people every day, but many people who work with gear up to about 500VAC have received across-the-chest shocks, then picked themselves up and carried on working.

Possibly with one hand behind their back.

### New look iBall?

I got a Zling V-Photo Vx for my little brother's birthday off eBay (I did ask him if he wanted something more well known). I think it's the same basic player as the Geil iBall, in a different shape.

Anyway, the driver package includes a utility to convert AVIs to the "MTV" format this player uses. Is that the same format the iBall plays (maybe change the extension)? This utility will do 16 frames per second.

David

The V-Photo certainly looks as if it's got the same hardware, doesn't it?

The "MTV" format isn't the same as the iBall's "MPV", though. It's got the same horrible file size increase problem as the Geil player's converter, but neither the default 128kbps-audio 16fps setting, nor 64kbps and 8fps (which matches the Geil software's only setting) worked, with changed suffixes.

Thanks for the attempt, though!

### Actually, I think He'dprobably be cool with it

At first I didn't believe "Students hear the word and hand in their CDs" could be serious.

It used to be that religious types thought perfectly good music should be destroyed because it was dangerous. Now they are destroying music – in order to protect it.

Somehow I think they miss the point in asking "what would Jesus do?" when they concluded that Jesus would probably think that music "piracy" is stealing.

Maybe I missed a bit in that loaves and fishes story, but I don’t seem to recall the bakers and fisherman being compensated for lost earnings due to the perfect mass reproduction and distribution of said loaves and fishes by Jesus.

Mark

The term "copyright theft" is an interesting one, because it has no meaning in law - or, at least, not at all the meaning it's given by the good Christian students in the SMH piece. A good Legal Studies teacher could have made an excellent lesson out of this issue - heck, maybe one has, by now. Such subversion is not entirely unknown in religious schools.

Copyright infringement is, of course, not "copyright theft", for all the usual tired reasons. The copyright owner has not been deprived of their property, merely of money they should, legally, have been paid - but probably never would have received anyway, particularly when the copier's a teenager with little disposable income.

"Copyright theft" is a real concept, though; it's the stealing of the copyright in a work from the person who rightfully owns it. A proper stands-up-in-court version of this would be cases of plagiarism, when someone copies someone else's work and receives compensation for it (compensation, in this case, can be money, an "A" on your copied essay, or just fame).

It can, however, also be argued that usurious music industry recording contracts are another example of copyright theft. Copyright in the music goes to the record company, and the artists get little or no compensation, according to ancient tradition.

I wouldn't be surprised if Jesus would recommend that the parents of these students stop paying the private school fees and send their kids to a State school, giving the money to the poor instead. This, and His further recommendations concerning luxury cars and large houses containing home theatre equipment, would probably fall on deaf ears.

I imagine Jesus would then go on, doggedly, to decry corporatism in general, including but not limited to record companies.

If asked about music copying, Jesus would probably request His audience stop bothering Him with trivia, and hold forth for a while about alleged Christians who support wars.

He might then lead an unlicensed rendition of a Woody Guthrie song or two.

### Option B: Upgrade to an M45

I was directed to your site by a friend who knows that I am interested in pyrotechnics. Now notice that I say INTERESTED, and not proficient. I have a great love of watching things explode or catch on fire and figured I would say hello.

The reason I am writing is to solicit some advice on a different form of pyrotechnics, namely propane. I have a hearse that I have outfitted with light armor and a flamethrower, Mad Max style, and I am looking to increase the flamethrower's range. I chose propane for the relative safety factor but I am having a difficult time getting a decent distance out of it. Any suggestions?

Zachary