why does a volt meter show voltage when you hold the probes in your hands?

[SonicIce]

say you set a multimeter to 2v DC and hold the probes tightly between your fingers in each hand. mine shows around .05v and jumps around. why?

 

[Paul98]

didn't you watch the matrix? Machines turn us into batteries! you got to watch out

 

[Evadman]

http://health.howstuffworks.com/human-body/cells-tissues/human-body-make-electricity1.htm

 

[Venom20]

The body is full of current. It's how things get communicated.

 

[Howard]

Your volt meter sucks.

 

[Red Squirrel]

You can actually get readings just from the air from all the radio waves and such. The current is VERY low though, so it would not really power anything.

A crystal radio will actually pickup enough current to power a low requirement earphone, actually. (I built one as a kid, was kinda neat, no batteries or anything)

 

[SonicIce]

Your volt meter sucks.

yea i got it free but they don't all do this?

 

[Jimmah]

yea i got it free but they don't all do this?

Correct.

 

[herm0016]

same way a heart monitor works. look up biopotentals.

 

[tcsenter]

say you set a multimeter to 2v DC and hold the probes tightly between your fingers in each hand. mine shows around .05v and jumps around. why?

Umm...because we're a little bit conductive?

 

[C1]

You can actually get readings just from the air from all the radio waves and such. The current is VERY low though, so it would not really power anything.

A crystal radio will actually pickup enough current to power a low requirement earphone, actually. (I built one as a kid, was kinda neat, no batteries or anything)

Yes, Art Bell used to tell about the energy that his HAM radio antenna would accumulate and the spark gap discharge that he would be able to create. (Art's antenna: http://www.smeter.net/w6obb/antenna-farm.php ).

Also remember the tether experiment in outer space. 1996, the Space Shuttle Columbia deployed an experimental tether into orbit. The experiment was called the Tethered Satellite System (TSS- 1R) and it's purpose of this was to attempt to generate electricity by utilizing Earth's magnetic field.

Art talked about the possibility of harnessing such [free] energy. Dont think he successfully implemented a design, but apparently some people have: http://www.eham.net/articles/24026

"This works as a charging system because the long antenna wire acts like a capacitor building an electric charge on the antenna wire. When a few thousand volts are reached, it will be discharge by "sparking" across the spark plug. The spark plug then delivers the electric charge to the coil, which in turn "down-converts" it to a few hundred volts. The electricity is then injected into the battery from the coil. The coil works by "pulsing" the charge into the battery at regular discharge intervals."

 

[Modelworks]

volt meters have a very high impedance so that they do not load down the device you are measuring. While the meter might read .05V the current is so low that it couldn't power the lowest current LED if you stored the charge for a year.

 

[TecHNooB]

people like to treat measuring devices as perfect :|

 

[evilspoons]

people like to treat measuring devices as perfect :|

In this case it's a perfectly valid measurement, there is potential difference across your skin.

That Discovery thing is reasonably accurate but then they start rambling about 25,000 volts to power a CRT and how eels are 600 volts. That's only half the picture, you need to know how much current can be delivered or is required.

It's like comparing a rock moving at 50 km/h to a brick traveling at 3000 km/h and saying the brick has more energy... the comparison is totally meaningless until you find out how heavy the rock and the brick are.

 

[C1]

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

 

[Red Squirrel]

people like to treat measuring devices as perfect :|

Well they are made to measure things, so one would hope they are decently accurate within say 10%. If not, what's the point?

 

[Paperdoc]

Well they are made to measure things, so one would hope they are decently accurate within say 10%. If not, what's the point?

Yes, it IS reasonable to expect "reasonably accurate". But we also must recognize that ALL measurement systems have limits on what they can do, and how accurate they can be under various circumstances. Measuring the voltage between your two hands is NOT what a standard voltmeter is designed for, so it is outside its capabilities to be accurate.

Nevertheless, what you see is partially true. You grab the two voltmeter probe ends, one in each hand, and you get a non-zero reading but it fluctuates. First you have to recognize that the meter is reading some voltage provided by a very high-resistance source. Now, the voltmeter itself actually is measuring a CURRENT through itself across its own input impedance and using an amplifier to do it. Knowing the current and the input impedance allows the meter to present to you on its display the result in terms of voltage. But if the impedance of the source (your body and hands) also is very high, the result can be inaccurate.

Why is there ANY non-zero voltage? Several possible sources, including things like induced currents from nearby AC fields, etc. But one VERY common one is: you have just created a battery of sorts. In electrochemistry look up Galvanic Cells. If you connect two pieces of metal to an external voltmeter and them place those two metals into some conductive medium (electrolyte), you have a Galvanic Cell. The most common ones we see everyday are batteries made with two DIFFERENT metals and some thick chemical paste between them, all sealed into a case. But another version is one in which the metals are the same, but the conductive medium in contact with each of them is different, and there is still some electrical connection between the two different media. Different how? Could be different types of electrolytes. But also, you get the same result if the electrolyte is fundamentally the same all over BUT the solution's concentration of certain dissolved components is a little different in the vicinity of the two metals. This is called a "Concentration Cell", and it is the basic mechanism by which metals can corrode - like when your car body starts rusting out in spots because of "road salt". Your car's body is mostly one type of metal, and the electrolyte is just wet salty deposits, but the concentration of the salt varies from one spot to another on the car. So you have a Concentration Cell and the Galvanic Cell system causes one area to oxidize the metal to soluble ions that leave - that's corrosion.

Now, back to you holding the two voltmeter probes. The moisture on your skin is an electrolyte - it contains a few slightly conductive dissolved materials. AND those materials are a little different between your two hands. So YOU are the battery, and the voltmeter is doing its best to show you the output. Now, the concentration of dissolved materials in each hand may vary a bit over time. ALSO, the resistance of the contact between your skin and the probe will vary over time as your hands flex. For both those reasons the voltmeter will actually "see" a varying voltage, and it will be very small since the source is a Concentration Cell driven by very small differences in the electrolytes (the skin moisture) at each metal probe.

 

[Modelworks]

The other reason a digital meter will show a reading when there isn't one is due to how they measure voltage internally. The circuits used are ADC and depending on the sampling rate you can see the voltage increase the longer you hold it because the frequency generated is causing the probes and the skin to work like a capacitor, slowly charging.


This wasn't an issue with analog meters. For those you just set it to ohms, crossed the probes and zeroed out the meter and were ready to go.