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Old 02-10-2006, 06:11 PM
Revenant Threshold Revenant Threshold is offline
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Will hair conduct electricity?

Watching Blade on TV, there's a scene in which a ( human ) woman is pushed down between train tracks on a metro/underground system. Her hair brushes against the electrified rail - but no other part of her body does.

What, in reality, would have happened to her? Suggestions have been that at the very least her hair should have caught fire. Answers, please?
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  #2  
Old 02-10-2006, 06:15 PM
CookingWithGas CookingWithGas is offline
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I just took an ohmmeter reading off my daughter's hair and got zero on the lowest scale. Hair does not conduct.

There's no reason that her hair would have caught fire if there's no current in it.

If it were wet that might be a different story.
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Old 02-10-2006, 06:19 PM
Q.E.D. Q.E.D. is offline
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Dry hair, no. Wet hair, probably to some extent. Everything is conductive to some degree, even the very best insulators. But at third rail voltages, which in the US are typically around 600V, the current which could be expected to pass along dry human hair would be negligible. With wet hair, it's possible she might have gotten a little jolt, but even then it wouldn't have been much--unless she'd been swimming in the ocean, or the Great Salt Lake.
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Old 02-10-2006, 07:04 PM
Mangetout Mangetout is offline
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Quote:
Originally Posted by CookingWithGas
I just took an ohmmeter reading off my daughter's hair and got zero on the lowest scale. Hair does not conduct.
Measuring resistance, zero on the lowest scale, on my multimeter theoretically indicates a perfect conductor; are you sure you don't mean 'out of range' on the highest scale?
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Old 02-10-2006, 07:13 PM
Bryan Ekers Bryan Ekers is online now
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Hair can conduct, to a degree. Edison experimented with hair as a filament in his early light bulbs.

Of course, the whole point is the filament offers so much resistance it glows hot and bright.
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Old 02-10-2006, 07:34 PM
CookingWithGas CookingWithGas is offline
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Quote:
Originally Posted by Mangetout
Measuring resistance, zero on the lowest scale, on my multimeter theoretically indicates a perfect conductor; are you sure you don't mean 'out of range' on the highest scale?
My bad. There was a short between the chair and the meter. I was using a multimeter and reading off the voltage scale instead of the ohms scale.

Should have said got a reading of infinity.
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Old 02-10-2006, 07:55 PM
JThunder JThunder is offline
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I wouldn't trust an ohmmeter or multimeter to measure the "resistance" of biological materials. Remember that these devices only generate a miniscule amount of voltage and current. Some materials conduct much more dramatically as the voltage is increased beyond a critical point.

In fact, the term "resistance" does not strictly apply to non-ohmic materials. Resistance is, strictly speaking, a constant. It only applies when you have a linear relationship between voltage and current... and for most materials, that's not the case. Such materials are more properly characterized by a transconductance curve--that is, a function (generally non-linear) that plots current against applied voltage.
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Old 02-10-2006, 08:18 PM
spingears spingears is offline
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Quote:
Originally Posted by Bryan Ekers
Hair can conduct, to a degree. Edison experimented with hair as a filament in his early light bulbs.
Wasn't his first step to carbonize the hair/filament to make it conductive?

Quote:
Of course, the whole point is the filament offers so much resistance it glows hot and bright.
It does? IIRC E-IR, R=E/I & I=E/R
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  #9  
Old 02-10-2006, 09:34 PM
Bryan Ekers Bryan Ekers is online now
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Quote:
Originally Posted by spingears
It does?
Relatively speaking, of course.

Anyway, resistance is inversely proportional to the size of the cross-section, so it's not just the type but the thickness of the conductor.

Incidentally, resistance increases as the filament heats up (the low resistance when the filament is cold invites a power surge that make it more likely to burn the filament when the bulb is intially powered). Tungsten can (usually) take the strain, but any dangerous current forced through hair will make it burn up before posing a shock hazard.
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Old 02-11-2006, 06:28 AM
Fridgemagnet Fridgemagnet is offline
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JThunder has a good point. At a high enough voltage, most things normally thought of as insulators (e.g. wood, many plastics) will become rather good conductors. The moisture content of hair might be enough to give a bit of a scalp tingle at 600V, and I wouldn't trust hair as an insulator for any higher voltage than this.

On an electrostatic front, I wonder which shampoo goes best with a van der Graaf generator?
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