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'[OT] Electrostatic sensing (was PIC-based angle se'
1997\12\09@160424 by Robert Nansel

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>In a message dated 97-12-09 11:11:30 EST, you write:
>
><< No, it's sensing the electric potential that naturally exists in the
> atmosphere, the same one responsible for lightning. With a sensitive
> instrumentation amp you can measure the voltage difference between your
> feet and your head (usually a couple hundred volts over six feet).
>  >>
>
>Is higher potential "always" up??
>
>Mark A. Corio
>Rochester MicroSystems, Inc.

If by "higher" you mean "more positive", then no. Clouds are negatively
charged with respect to the ground by as much as 1000 megavolts.
Precipitation and wind currents act like the charge transport belt of a Van
de Graaff generator (except Van de Graaf generators usually are designed to
produce a positive charge on the top electrode). The charge persists
because air is such a good insulator.

--BN



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1997\12\10@134858 by Bruce Cannon

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>With a sensitive instrumentation amp you can measure the voltage
difference >between your feet and your head (usually a couple hundred volts
over six feet).

This is really interesting!

But if it's a couple of hundred volts, why do you need a sensitive
instrument amp?

Bruce

1997\12\10@140534 by Tim Kerby

picon face
<x-rich>Don't you mean a few hundred mV.  I produce 0.5V hand to hand.  Higher
voltages are only produced in long <bold>conductors</bold> like wire
because of atmospheric potential and moving through the earths magnetic
field to a much lesser extent.


I once was caught flying a kite in the rain with a thunderstorm five
miles away.  As I took the kite in I had to wear gloves as the 200 foot
of line gave me huge electric shocks!



Tim Kerby


At 10:13 10/12/97 -0800, you wrote:

>>With a sensitive instrumentation amp you can measure the voltage

>difference >between your feet and your head (usually a couple hundred
volts

{Quote hidden}

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1997\12\10@154451 by Robert Nansel

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>Don't you mean a few hundred mV.  I produce 0.5V hand to hand.  Higher
>voltages are only produced in long conductors like wire because of
>atmospheric potential and moving through the earths magnetic field to a
>much lesser extent.

Nope. Volts, not millivolts. Depends on the weather, of course, but, as
anybody who works with radio towers knows, it's an easily observed
phenomenon.

--BN


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  69 S. Fremont Ave. # 2     for education and industry"
  Pittsburgh, PA 15202
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1997\12\10@154459 by Robert Nansel

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face
>>With a sensitive instrumentation amp you can measure the voltage
>difference >between your feet and your head (usually a couple hundred volts
>over six feet).
>
>This is really interesting!
>
>But if it's a couple of hundred volts, why do you need a sensitive
>instrument amp?
>
>Bruce

Actually, it's not that it has to be all that sensitive, but high impedance
and high CMRR is a must. Basically you are building an electrometer. Then
there's the trouble with static buildup. Without something like the
radioactive sources mentioned previously in this thread to dissipate charge
buildup, the sensor would quickly saturate and become unresponsive to
changes in the voltage potential in the environment.

I wonder, though, whether you could get around the need for radioactive
sources by periodically shorting your sense and reference plates together.
Anybody out there know?

--BN



----------------------------------------------------------------------------
  Country Robot            "Modular robot components
  69 S. Fremont Ave. # 2     for education and industry"
  Pittsburgh, PA 15202
----------------------------------------------------------------------------

1997\12\10@162432 by Martin McCormick

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Bruce Cannon writes:
>difference >between your feet and your head (usually a couple hundred volts
>over six feet).
>
>This is really interesting!
>
>But if it's a couple of hundred volts, why do you need a sensitive
>instrument amp?

       You need that because this voltage has almost no current at all,
thank goodness.:-)   Any conductivity in your measuring device will suck
down the potential to nothing so that one will not see it.

       Very high-impedance amplifiers will let enough potential build up
to detect.

       The gradient varies with humidity and weather conditions.  It is even
known to reverse polarity during thunderstorms and lightning is simply
this gradient carried to extreme so a strong dose of common sense is in order
if one wants to experiment with it.

       I once had a wire amateur radio antenna about 20 feet above ground
and forgot to connect it to Earth when a thunder head began to rumble in the
distance.  The lightning was many miles away, but the static gradient was
so high that sparks began arcing the approximately 8-millimeter distance between
the center conductor of the coaxial cable connected to the wire and the
shell of that connector which was ground.  It was just like listening to
an electronic fuel igniter.  I grounded the connector, but one should never
wait until things get that far along before doing something about them.
What I did was flat dangerous and stupid.  It was also really neat to observe.

       There is probably not that much to worry about on a clear day except
for damage to FET's, but be sure to look out for
unsettled weather so you can live to tell about it.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1997\12\10@163347 by Martin McCormick

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Robert Nansel writes:
>I wonder, though, whether you could get around the need for radioactive
>sources by periodically shorting your sense and reference plates together.
>Anybody out there know?

       I don't know why not.  Some people have connected neon lamps and
fluorescent tubes between their pickup wires and Earth which causes the
gas tube to flash each time the potential reaches whatever voltage is the
breakdown voltage for that lamp.  The ionized gas then discharges the
potential until it gets so low that the gas is no longer ionized at which
time it starts to build up again; (sort of lightning in a bottle.)  One could
measure the time between flashes and determine whether the potential was
increasing or decreasing.

Martin McCormick

1997\12\10@183708 by Mcorio

picon face
In a message dated 97-12-10 14:14:07 EST, you write:

<< >With a sensitive instrumentation amp you can measure the voltage
difference >between your feet and your head (usually a couple hundred volts
over six feet).

This is really interesting!

But if it's a couple of hundred volts, why do you need a sensitive
instrument amp?

Bruce
 >>
Also, if this is true (now that we are thinking about it) why do we not notice
it in this case, but if we put a 115V mains on our head and stood on a copper
pipe to earth we would be paramedic material??

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
****** Designing Electronics for Research and Industry ******

1997\12\10@184535 by Sean Breheny

face picon face
At 06:22 PM 12/10/97 EST, you wrote:
>In a message dated 97-12-10 14:14:07 EST, you write:
>
><< >With a sensitive instrumentation amp you can measure the voltage
> difference >between your feet and your head (usually a couple hundred volts
> over six feet).
>
> This is really interesting!
>
> But if it's a couple of hundred volts, why do you need a sensitive
> instrument amp?
>
> Bruce
>  >>
>Also, if this is true (now that we are thinking about it) why do we not
notice
>it in this case, but if we put a 115V mains on our head and stood on a copper
>pipe to earth we would be paramedic material??
>

Hi Mark,

The reason why we are not harmed by this electric field gradient is that ,
as someone else mentioned before, the current involved is so low. The
voltage comes from just a few electrons difference in charge, separated by
a very good insulator. If we bridge the gap, only a tiny current flows. It
is sort of like a static discharge which occurs when one walks  accross a
carpet and touches a doorknob. The charge transfer is bearly enough for you
to feel it (or sometimes, it is quite enough for you to feel it, but it is
for such a short duration that it does you no harm). In fact, the voltages
involved in these little shocks gotten by touching a door knob can easily
be in the megavolts, or so I have read.

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
.....shb7KILLspamspam@spam@cornell.edu
Phone(USA): (607) 253-0315

1997\12\10@214332 by Robert Nansel

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face
>In a message dated 97-12-10 14:14:07 EST, you write:
>
><< >With a sensitive instrumentation amp you can measure the voltage
> difference >between your feet and your head (usually a couple hundred volts
> over six feet).
>
> This is really interesting!
>
> But if it's a couple of hundred volts, why do you need a sensitive
> instrument amp?
>
> Bruce
>  >>
>Also, if this is true (now that we are thinking about it) why do we not notice
>it in this case, but if we put a 115V mains on our head and stood on a copper
>pipe to earth we would be paramedic material??
>

Because there is almost no energy stored in the charge. Air--unionized, at
least--is an excellent insulator, but a pretty lousy capacitor for energy
storage (near infinite ESR!).

--BN


----------------------------------------------------------------------------
  Country Robot            "Modular robot components
  69 S. Fremont Ave. # 2     for education and industry"
  Pittsburgh, PA 15202
----------------------------------------------------------------------------

1997\12\11@091203 by Mcorio

picon face
In a message dated 97-12-10 19:19:10 EST, you write:

<<
The reason why we are not harmed by this electric field gradient is that ,
as someone else mentioned before, the current involved is so low. The
voltage comes from just a few electrons difference in charge, separated by
a very good insulator. If we bridge the gap, only a tiny current flows. It
is sort of like a static discharge which occurs when one walks  accross a
carpet and touches a doorknob. The charge transfer is bearly enough for you
to feel it (or sometimes, it is quite enough for you to feel it, but it is
for such a short duration that it does you no harm). In fact, the voltages
involved in these little shocks gotten by touching a door knob can easily
be in the megavolts, or so I have read.

Sean
 >>
If the charge is so low (i.e. only a small current available) why is it not
discharged quickly and therefore no large gradient is maintained. I do not
disagree with your description, but I am still not sure about the contradition
with the mains-to-the-head analogy.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
****** Designing Electronics for Research and Industry ******

1997\12\11@105950 by paulh

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On Thu, 11 Dec 1997, Mcorio wrote:

> If the charge is so low (i.e. only a small current available) why is it
> not discharged quickly and therefore no large gradient is maintained. I
> do not disagree with your description, but I am still not sure about the
> contradition with the mains-to-the-head analogy.

You're right.  It works like a Van DeGraf generator.  If you short it
out, you won't measure a voltage.  The voltage gradient between my head
and my toes is pretty darn small, because I'm a pretty good conductor.
However, the voltage gradient between 2 points outside separated
vertically by 6 feet is a few hundred volts.

The trick is measuring it, without shorting it out.  My cheap digital
voltmeter sees a few hundred millivolts if I lower one electrode and raise
the other, but I'm having trouble clipping the leads to the air (-:.  If I
clip each lead to a couple chunks of aluminum foil, then the measured
voltage goes up.  If I short it out, the voltage drops to zero.  When I
remove the short the voltage climbs.  The bigger the electrodes, the
faster it moves.

My meter sucks for this sort of experiment.  It's highest impedance is on
the millivolt range.  If the impedance didn't change, then it was
registering 300 millivolts on the millivolt range, it should register .3
if switched to volts.  Instead it sees .005 volts.  The voltmeter loads
the leads to heavily with only a couple of square feet of aluminum foil
for contacts.  I guess I need a better voltmeter or 20 foot long
uninsulated wires.  20 foot long wires are not practical on RC airplanes.

Some clever person figured out that ionized air is a conductor.  If you
ionize the air near the contacts with a little bit of radioactive
material, it's as if the contacts were many many times larger.

===Now I'm going Way Way Off Topic, but I can't help myself.=====

I want the thread to die, so I shouldn't mention this.  But it was too
cool not to note.  With the meter hooked up to the foil, I could detect a
running cat at 15 feet.  It is dry here this time of year, and he is a
long haired cat.  I imagine he is really charged up.  He enjoys chasing
the dot from a laser pointer.  So even though he is a cat, I could get him
to run anywhere I wanted, when I wanted, to do experiments.

If I was still, and the cat was far away, the voltmeter showed a few
millivolts.  If I had the cat run towards the negative meter lead, the
meter would register a few volts.  When he ran away, it would go negative.
At longer distances, I'd see smaller changes, but he could still change
that last digit at about 15 feet.  Since the measured voltage went up as
he approached the negative probe, and down when he went away, he's got a
significant negative charge.

I can't repeat the experiment with my other cats, they don't chase the
laser dot.

--
paulhspamKILLspamhamjudo.com  http://www.hamjudo.com
The April 97 WebSight magazine describes me as "(presumably) normal".

1997\12\11@111638 by Paul BRITTON

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Paul Haas <.....paulhKILLspamspam.....HAMJUDO.COM> wrote:

>Subject:      Re: [OT] Electrostatic sensing (was PIC-based angle sensor)
.
.
.

>
>===Now I'm going Way Way Off Topic, but I can't help myself.=====
>
>I want the thread to die, so I shouldn't mention this.  But it was too
>cool not to note.  With the meter hooked up to the foil, I could detect a
>running cat at 15 feet.  It is dry here this time of year, and he is a
>long haired cat.  I imagine he is really charged up.  He enjoys chasing
>the dot from a laser pointer.  So even though he is a cat, I could get
him
>to run anywhere I wanted, when I wanted, to do experiments.
>
.
.
.
So could you have a PIC controlled laser pointer........, and tie a
lawnmower to the back of the cat......and ...shall I stop now?

                       Paul!

1997\12\11@113448 by John Shreffler

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part 0 1503 bytes
-----Original Message-----
From:   Mcorio [SMTP:EraseMEMcoriospam_OUTspamTakeThisOuTAOL.COM]
Sent:   Thursday, December 11, 1997 9:04 AM
To:     PICLISTspamspam_OUTMITVMA.MIT.EDU
Subject:        Re: [OT] Electrostatic sensing (was PIC-based angle sensor)

In a message dated 97-12-10 19:19:10 EST, you write:

<<
The reason why we are not harmed by this electric field gradient is that ,
as someone else mentioned before, the current involved is so low. The
voltage comes from just a few electrons difference in charge, separated by
a very good insulator. If we bridge the gap, only a tiny current flows. It
is sort of like a static discharge which occurs when one walks  accross a
carpet and touches a doorknob. The charge transfer is bearly enough for you
to feel it (or sometimes, it is quite enough for you to feel it, but it is
for such a short duration that it does you no harm). In fact, the voltages
involved in these little shocks gotten by touching a door knob can easily
be in the megavolts, or so I have read.

Sean
 >>
If the charge is so low (i.e. only a small current available) why is it not
discharged quickly and therefore no large gradient is maintained. I do not
disagree with your description, but I am still not sure about the contradition
with the mains-to-the-head analogy.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
****** Designing Electronics for Research and Industry ******

1997\12\11@170759 by Tim Kerby

picon face
Hi
Why do you think I have started a thread on controlling a laser with the
pic then?
I have a cat that chases my pointer too!

Tim Kerby


At 16:12 11/12/97 +0000, you wrote:
{Quote hidden}

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1997\12\11@171006 by XYGAX

picon face
In a message dated 11/12/97  17:23:43, you write:

<< So could you have a PIC controlled laser pointer........, and tie a
lawnmower to the back of the cat......and ...shall I stop now?

                        Paul!
 >>
No No No
Its a GOAT cats need an external interface to mow the lawn........

Sorry Steve....

1997\12\12@053105 by paulb

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face
Sean Breheny wrote:

> In fact, the voltages involved in these little shocks gotten by
> touching a door knob can easily be in the megavolts, or so I have
> read.

 Kilovolts, actually.  These sparks are three or four millimetres long.
Played with the Ultor connector from a B/W TV?  Half to an inch; ten to
twenty kilovolts.  Not recommended with colour TVs.  A "mini" Van der
Graaf generator about ten inches; 150 to 200kV.  Megavolts (mains line
testing stations including really BIG Van der Graaf generators) and you
are looking at six foot bolts.

 Very approximate, depends on humidity and contact shape.  Points
produce such a "spray" of discharge that they won't allow a high voltage
to be accumulated at all.  And Van der Graaf generators are now obsolete
in favour of Cockroft-Walton ("ladder") multipliers using comparatively
cheap semiconductor High Voltage rectifiers.

 An ionisation source forms a sort of "very sharp" discharge point.  I
hadn't considered this before.  My experience of cheap smoke detectors
is that they are rubbish, a couple I bought and stored for a year
insisted on beeping intermittently even though I could not fault the
batteries and even put in new ones.  Others lasted a few years then
karked.  Is this other people's experience too?  I'm not impressed, but
now I have some ideas on what to do with the old ones!

 Cheers,
       Paul B.

1997\12\15@182431 by rustyc

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face
Martin McCormick wrote:

>         I once had a wire amateur radio antenna about 20 feet above ground
> and forgot to connect it to Earth when a thunder head began to rumble in the
> distance.  The lightning was many miles away, but the static gradient was

Been there, done that too.  Twas a 2m beam antenna (4 el - pretty small), up 20
feet
also.
(In Colorado, Lafayette to be exact).  I shorted the coax and threw it out the
window. (After
the sparks - including one or two pretty nasty shocks...)  Storm had enough
close
hits to blow out my modem!  (Of course, I foolishly tried to USE the stupid
thing
during the storm - yikes)

rusty

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