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'[OT] Cable retained energy, was Super Caps'
1998\03\10@051542 by Jorge Ferreira

flavicon
face
       Hi Alex



At 12:28 98.03.06 -0800, you wrote:
{Quote hidden}

to run
{Quote hidden}

When you test the insulation of a long spool of cable (one end connected to
a power source and the other open), when you disconect it from the power
source and leave it open the transient efect can creat a surge at one end
of the cable (the one connected with the power source). As the cable is
open at both ends this surge travels from one end to the other (as a
reflectd wave) until all its energy is exausted by the cable losses. If you
have a good quality cable this energy can stay there for quite a while.
If we are talking about high voltage cables (energy transportation lines),
this efect can kill pepople toutching the cable days after it has been tested.
The only safe way to avoid this is to conect the cable to a correctly
adapted load until all residual energy gets exausted before stocking the
cable.

       Jorge F

===============================================================
cumprimentos / best regards
     Jorge Ferreira          //spam_OUTjorgegfTakeThisOuTspammail.telepac.pt
------ Make sure brain is in gear before engaging mouth -------
===============================================================

1998\03\10@152159 by Scott Newell

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face
>When you test the insulation of a long spool of cable (one end connected to
>a power source and the other open), when you disconect it from the power
>source and leave it open the transient efect can creat a surge at one end
>of the cable (the one connected with the power source). As the cable is
>open at both ends this surge travels from one end to the other (as a
>reflectd wave) until all its energy is exausted by the cable losses. If you
>have a good quality cable this energy can stay there for quite a while.
>If we are talking about high voltage cables (energy transportation lines),
>this efect can kill pepople toutching the cable days after it has been
tested.
>The only safe way to avoid this is to conect the cable to a correctly
>adapted load until all residual energy gets exausted before stocking the
>cable.

Are you saying that the resistance of the cable is so low that a signal can
bounce along the unterminated transmission line for _days_ without
undergoing significant attenuation?  (If so, wouldn't that essentially be a
superconducting tuned transmission line oscillator?)


Isn't it _much_ more likely that the cable (I'm assuming we're discussing
coax or two conductor cable) is acting simply as a capacitor?  After all,
what's two conductors seperated by an insulator?


newell

1998\03\10@184926 by Sean Breheny

face picon face
At 02:20 PM 3/10/98 -0600, you wrote:
{Quote hidden}

I find either case (the "superconductivity" or the capacitance theory)
unlikely under normal circumstances. If it did happen, I would attribute it
to the capacitance.
       First off, Scott (or Newell, sorry, still not sure which you prefer) is
right, as far as I know, that no ordinary conductor can possibly support
currents for more than a millisecond or so with out an external source.
(even an ms is an EXTREAMLY long estimate).
        Secondly, lets suppose that there was enough wire in the cable to creat
e
a significant capacitance. The effective circuit would look like a
capacitor with a resistance in parallel (we can ignore the inductance, it
certainly won't make much difference beyond seconds of time). The value of
the effective parallel resistance can be approximated by the resistance of
the cable. Assuming the cable is copper and of a resonable diameter, lets
say 26 ga. 26 ga copper wire has a linear resistivity of .0408 ohm/ft. So,
even if this cable were 10 miles long, the effective resistance would only
be around 2.2k. Now, suppose we had 1 farad of capacitance. The time
constant for the circuit would be 2200 seconds. So, the voltage would drop
to less than 1% of its original value within hours of charging, and this is
for an extream example. The spool of wire holding 10 miles of wire would
probably barely fit on my desk. So, I guess if you had a huge spool, you
could theoretically have it hold charge for days or weeks, but it would
have to be HUGE. Besides, this effect diminishes with wire diameter so
anything which one could rightfully call "cable" instead of wire would
probably not exhibit this effect at all.
       In order for a static charge to naturally build up on the cable, the
average rate of energy transfer to the cable would have to exceed its
discharge rate. This also seems unlikely. However, I have no experiance
dealing with any really large amount of wire or cable, so I certainly could
be wrong.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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1998\03\10@192333 by Jorge Ferreira

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At 14:20 98.03.10 -0600, you wrote:
{Quote hidden}

In normal low power aplications, all the energy is lost in fractions of a
second, but in cables for energy transportation lines who work at very high
voltages (several KV) and low freq the insulation testing is above the
double of the nominal working voltage, so if you have 1000 meters of cable
its a pretty good amount of energy we are talking about and in this case
you can get killed days after the testing if that energy isnt properly
drained.
About beeing a capacitive efect, I think you can call it that. We shouldn't
forget that one of the matematical models for a cable (or trnsmition line)
is a RL series circuit with caps in paralel.
                 R            L
       ------/\/\/\/\----@@@@@-----
                       |
                       |
                      ---
                      --- C
                       |
                R      |    L
       ------/\/\/\/\----@@@@@-----

(hope you can understand this, first time, ASCII line art)


       Jorge F
===============================================================
cumprimentos / best regards
     Jorge Ferreira          //jorgegfspamKILLspammail.telepac.pt
------ Make sure brain is in gear before engaging mouth -------
===============================================================

1998\03\10@203512 by Scott Newell

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face
>         Secondly, lets suppose that there was enough wire in the cable to
create
>a significant capacitance. The effective circuit would look like a

RG-58 is about 30 pF/foot, according to my Belden catalog.


>capacitor with a resistance in parallel (we can ignore the inductance, it
>certainly won't make much difference beyond seconds of time). The value of
>the effective parallel resistance can be approximated by the resistance of
>the cable. Assuming the cable is copper and of a resonable diameter, lets

Sorry, if you take a hunk of coax with both ends open, any parallel
resistance _across_ the cap has to be from the insulation resistance of the
dielectric.

I don't have any teflon coax, but I'll bet it makes a _very_ good
low-leakage cap.

Look, at low frequencies, the capacitance of coax is like any other cap.
It stores a charge.  Put enough voltage across that cap and it can bite
you.  Use a high-quality dielectric and the stored charge will be there for
quite some time.


>for an extream example. The spool of wire holding 10 miles of wire would
>probably barely fit on my desk. So, I guess if you had a huge spool, you

I've got a spool that's about 26 miles long, and it isn't very big.  It's
not coax, and it's only 34 gauge.  Too bad I don't have access to both
ends--it'd be a neat inductor!


newell

1998\03\10@214541 by Sean Breheny

face picon face
At 07:34 PM 3/10/98 -0600, you wrote:
>>         Secondly, lets suppose that there was enough wire in the cable to
>create
>>a significant capacitance. The effective circuit would look like a
>
>RG-58 is about 30 pF/foot, according to my Belden catalog.
>
>

Ok, maybe I was getting confused, I thought we were talking about single
conductor wire (loosely called cable). Of course, my whole discussion would
be incorrect for coax. Jorge has mentioned that this phenomenon is
observable when dealing with power transmission line. Power transmission
line is not coaxial, is it? I have always been told that it is single
conductor woven cable, sometimes with a hollow core with a coolent run
through it.

{Quote hidden}

Yes, I understand this, but as I said above, I thought that you meant
single conductor. I think the original author was refering to single
conductor wire, or at least was initially interpreted as such. The reason
for the resistance in parallel with the cap. is that you can view a single
wire as split up into elements, each one forming a plate of a capacitor
with adjacent elements as the opposite plates, and each little element
forming a little resistance. I then mentally integrate over the length of
the cable to form an electrical picture of what is going on.

>>for an extream example. The spool of wire holding 10 miles of wire would
>>probably barely fit on my desk. So, I guess if you had a huge spool, you
>
>I've got a spool that's about 26 miles long, and it isn't very big.  It's
>not coax, and it's only 34 gauge.  Too bad I don't have access to both
>ends--it'd be a neat inductor!
>

Wow, I bet it would be! Even better if you stuck a core in it!

>
>newell
>
+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
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1998\03\10@231500 by Mike Keitz

picon face
On Tue, 10 Mar 1998 21:38:31 -0500 Sean Breheny <EraseMEshb7spam_OUTspamTakeThisOuTCORNELL.EDU>
writes:
> Jorge has mentioned that this phenomenon is
>observable when dealing with power transmission line. Power
>transmission
>line is not coaxial, is it?

The stuff that is buried underground and used from about 2 KV up to maybe
50 KV (e.g. for distribution to transformers in neighborhoods) is.  It
has a copper or aluminum conductor at the core, thick polyethylene
insulation, then a shield made out of metal braid and conductive plastic,
and finally a black plastic outer covering.  The shield is likely not
intended to normally carry current.  It is connected to ground at both
ends for safety reasons.  Maybe some single-phase circuits use it as the
return conductor.  In any case, the coaxial construction makes it
difficult for high voltage to appear on the outside of the cable if there
is a leakage, cut or hole in the insulation.  Such a circumstance will
cause the cable to arc and the fuse or circuit breaker to open.  This
line is of course routinely "hipot" tested before installation by
connecting a high voltage between the center and the shield and measuring
any leakage.  A big spool of it charged to many KV would indeed be
dangerous.  The polyethylene insulation would prevent the charge from
leaking off for a long time as well as maybe causing a phantom "recharge"
from dielectric absorption.

Of course such cable is intended for use at 60 Hz and is not designed for
any particular impedance.

>I have always been told that it is single
>conductor woven cable,

The wire strung overhead between poles is laid up of strands of aluminum
on the outside, and steel on the inside.  The steel is to make it
stronger.  Even at 60 Hz, there is enough of a "skin effect" to reduce
the effectiveness of the conductors in the center, so the aluminum is
outside.   Except in special cases, it is bare.  Please do not climb
power poles and touch the wires.  You will get hurt.  Really high power
lines have several cables in parallel.  They are held apart in a triangle
or square arrangment to make the apparent diameter of the conductor as
large as possible.   This improves the impedance and current distribution
along the line.  When dealing with hundreds of km of cable, AC effects
such as phase shifts are very real at 60 Hz.

sometimes with a hollow core with a coolent run
>through it.

In power "substations" the conductors are sometimes placed inside much
larger outer pipes.  The pipes are filled with sulfur hexafluoride gas.
SF6 is a much better insulator than air, as well as being certain to be
clean and dry, so the conductors can be in closer proximity than if they
were strung out in the open.  This is rather expensive so it is done only
for extremely high voltages or where the real estate to lay out the
station with larger seperations would be even more expensive.

I don't think the conductors in transmission systems are hollow or
actively cooled in any way.  Hollow cooled wire is used to wind
generators and big motors.  Hydrogen (H2) gas was used as the coolant in
one example I saw.  It's both very "thin" and light (i.e. easy to move at
high velocity through small hollow passages) and absorbs heat well.  It
doesn't seem that H2 gas and big electrical machines should be used near
each other.

This is what I remember from several years ago; current practices may be
different.

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1998\03\11@033328 by Rick Dickinson

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At 06:47 PM 3/10/98 -0500, Sean Breheny wrote:
>         Secondly, lets suppose that there was enough wire in the cable to
create
>a significant capacitance. The effective circuit would look like a
>capacitor with a resistance in parallel (we can ignore the inductance, it
>certainly won't make much difference beyond seconds of time). The value of
>the effective parallel resistance can be approximated by the resistance of
>the cable.

Sorry, Sean.  The resistance in parallel would be the resistance of the
insulator between the two conductors, not the resistance of the wire,
itself.  This is typically *very* high.  To make calculation easier, it is
typically specified as a certain conductance per unit length (conductance
is just the reciprocal of resistance).

- Rick "micro-mhos per meter" Dickinson
+---------------------------------+---------------------------+
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1998\03\11@061302 by richard skinner

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The cable he used was clx, I think.  It is built almost like coax.  It can
have any number of pairs in it.
Each individual wire inside  consists of a core, insulator, Wrapped with a
thin layer of foil.  Some of this
wire has anywhere from a single conductor to say 12 conductors in side.  The
very outside has a dielectric
type of grease before the final sheath or insulation.  Nice wire, used
offshore on platforms all the time, however,
it is a pain to work with.  When it gets old, it is tough to peel the foil of
the outside of each strand, then ofcourse,
the grease sure doesn't help matters any trying to tape it or grip it.

Static Electricity?  I don't know, can it be created by salt water waves
sloshing against the legs of the platform?
Could it be parasitic charges from the generators on board?  Geez, I have no
idea.  I know that it should definetly
have a good ground since its sitting on the ocean flooring with a ton of dirt
or mud holding it in place.

The cable was intentionally charged and then discharged to prove to me it
would hold a charge, and to prove to
me it must be discharged before handling.  It had to be proved as I though my
leg was being pulled.

I have seen formulas here saying that it is not possible, and I've seen
people saying yes it was possible for these
spools to hold a charge, All I know, and I have NO reason to make up short
stories on spools of wire, is he proved
it to me by doing it.

It did seem like an interesting debate,

Richard Skinner
@spam@rwskinnerKILLspamspamworldnet.att.net
http://home.att.net/~rwskinner

----------
{Quote hidden}

1998\03\11@063540 by Russell McMahon

picon face
This seems to make the most sense - the construction you
describe sounds like you potentially have several separate
conductors with the potential (pun unintended) to become
charged. Someone else had a figure for capacitance betwixt
wires in a pair - a figure of around 70nF/mile for
multi-pair telecommunications cable (way back then, we're
metric now) rings a bell. That is between conductors in  one
pair. Lots of pairs will have lots of separate capacitors.
Someone else can work out the energy stored (1/2CV^2) - its
late here in tomorrow so I'm off to bed.

{Original Message removed}

1998\03\11@081509 by Sean Breheny

face picon face
At 12:17 AM 3/11/98 -0800, you wrote:
>
>Sorry, Sean.  The resistance in parallel would be the resistance of the
>insulator between the two conductors, not the resistance of the wire,
>itself.  This is typically *very* high.  To make calculation easier, it is
>typically specified as a certain conductance per unit length (conductance
>is just the reciprocal of resistance).
>
> - Rick "micro-mhos per meter" Dickinson

Hi Rick,

I thought that the original questioner was referring to a single conductor
wire, not coax. For coax, of course my calculation would not be correct.

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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1998\03\11@131602 by Rob

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face
On Wed, 11 Mar 1998, richard skinner wrote:

>
> The cable was intentionally charged and then discharged to prove to me it
> would hold a charge, and to prove to
> me it must be discharged before handling.  It had to be proved as I though my
> leg was being pulled.

I'm sorry I missed some of this discussion but I was wondering how he
proved t held a charge?  How did he charge it (voltage level, type of
instrument, lenght of time...etc)?  How did he show it held a charge
(voltmeter)?  How did he discharge it (short, resistor leak path)?
Where was the cable during the test (rolled, laid out..etc)?  Did he show
the cable was at close to no voltage after discharge?

I have heard of antenna wires showing charges, but this was because of
the natural EMF of the background.

Just curious.

Rob

{Quote hidden}

1998\03\11@184606 by Reginald Neale

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face
Rob said:

>I'm sorry I missed some of this discussion but I was wondering how he
>proved t held a charge?  How did he charge it (voltage level, type of
>instrument, lenght of time...etc)?  How did he show it held a charge
>(voltmeter)?  How did he discharge it (short, resistor leak path)?
>Where was the cable during the test (rolled, laid out..etc)?  Did he show
>the cable was at close to no voltage after discharge?
>
>I have heard of antenna wires showing charges, but this was because of
>the natural EMF of the background.
>
>Just curious.
>
I too am curious about the the specs on this cable. As a reference, a small
camera flash stores about 5 joules of energy. That's enough to make some
decent sparks, and more than enough to be dangerous. The formula for
capacitive energy storage is [C*V^2]/2. As an earlier poster mentioned,
RG-58 is approx 30pf/ft. When you do the math, you'd have to charge about
666 ft to about 1KV (probably wouldn't handle that) or 66666 ft to 100V to
get 5 joules. That's 11 miles of cable. Or maybe this cable is something
really exotic... always possible, given the eclectic nature of PicLister
backgrounds.

Reg Neale

1998\03\11@224156 by richard skinner

flavicon
face
Okay, Okay, Next time I go out to the platform, I will get you the specs on
the cable.
It shouldn't be too long as I get to live there most of the time anyways.

And, to answer the question how it was discharged, it was shorted (touched
against)
some plate steel, and it made a small arc.  It was charged probably a few
minutes,
directly removed and shorted.  I do not know the voltage, but I will ask more
when I go out.
It couldn't have been tooo much as The most I "think" they have on site is
480 vac 3 phase.

Yes, it was still spooled on the spool.  I will also get the length, I'm sure
its written on the spool.
The spool was approx. 3 feet in diameter, CLX Wire, and It was mutiple
conductor (want me
to find that out too)?

It wasn't intended as a scientific experiment, but to show me and others,
that sometimes
things "can" get you when you least expect it.

I was being a smart ass about powering a clock from it, but the main content
was if anyone knew,
or seen this trick before, as I never expected such.

Geez, I think Ryan was right about people being very agressive on the list
here.  I made a mistake to begin
with, I thought I posted to the stamp list, but mistakenly posted here.  If I
was making this crap up, I would
have bailed out when I had the chance?


Richard Skinner
rwskinnerEraseMEspam.....worldnet.att.net
http://home.att.net/~rwskinner

----------
{Quote hidden}

1998\03\11@232106 by Rob Zitka

flavicon
face
At 09:22 PM 3/11/98 -0600, you wrote:

Hey Richard, I didn't mean to ruffle your feathers.  I was just very
curious to the phenomenon you saw.  As an engineer I like to try to
understand what goes on.  I appreciate what you've told us.  If you decide
to not respond that's fine.

Rob

{Quote hidden}

1998\03\12@043016 by Russell McMahon

picon face
If capacitive storage is the mechanism here, as I believe it
is, then the energy may be seen to be high enough to be
"impressive" as follows
Use 30pf/ft = 158 nF/mile - say 160nF - about double what I
recalled for buried telephone communications cable PER
PAIR..

Imagine that there are about 6 separate wires - the
description of the cable suggested the possibility of a
large number of multiple cores (I choose 6 for obvious
reasons - choose your oiwn figures to suit). 6 x 160 =
approx 0.1uF. Charge this to 100 volts.

Now, any volunteers from amongst the ranks of the
electrically experienced to seize a 0.1uF cap charged to 100
volts? At higher, say 500 volts, there will be even fewer.
Surprisingly, even at 500 volts we only have a small
fraction of a joule here (0.5 x 0.1E-6 x 500^2) and yet
practical experience tells you you are going to get a nasty
shock from it. (Or, if it doesn't, grab hold and see :-)).
You would have to grab the ends of all pairs at once,
logistically difficult, but the danger is a real one.

I am vaguely disturbed by the thought that N wires in close
proximity may not produce N times the capacitance but no
doubt someone will comment on this (too lazy to go and find
textbook on fundamentals to refresh memory).

>I too am curious about the the specs on this cable. As a
reference, a small
>camera flash stores about 5 joules of energy. That's enough
to make some
>decent sparks, and more than enough to be dangerous. The
formula for
>capacitive energy storage is [C*V^2]/2. As an earlier
poster mentioned,
>RG-58 is approx 30pf/ft. When you do the math, you'd have
to charge about
>666 ft to about 1KV (probably wouldn't handle that) or
66666 ft to 100V to
>get 5 joules. That's 11 miles of cable. Or maybe this cable
is something
>really exotic... always possible, given the eclectic nature
of PicLister
>backgrounds.
>
>Reg Neale
>

1998\03\12@060240 by richard skinner

flavicon
face
Sorry Rob, I was a little on edge last night to begin with, then after
reading all the flames,
I started to overheat a little.  At any Rate, When I go out to the platform,
I will get the info
off the spool for everyone.  The Nice thing about this wire is, it seems to
last forever (a matter of speaking)
through some pretty rough conditions.

Richard Skinner
RemoveMErwskinnerKILLspamspamworldnet.att.net
http://home.att.net/~rwskinner

----------
{Quote hidden}

If I
{Quote hidden}

show
> >> >the cable was at close to no voltage after discharge?
> >> >
> >> >I have heard of antenna wires showing charges, but this was because of
> >> >the natural EMF of the background.
> >> >
> >> >Just curious.
> >> >
> >> I too am curious about the the specs on this cable. As a reference, a
small
> >> camera flash stores about 5 joules of energy. That's enough to make some
> >> decent sparks, and more than enough to be dangerous. The formula for
> >> capacitive energy storage is [C*V^2]/2. As an earlier poster mentioned,
> >> RG-58 is approx 30pf/ft. When you do the math, you'd have to charge
about
> >> 666 ft to about 1KV (probably wouldn't handle that) or 66666 ft to 100V
to
> >> get 5 joules. That's 11 miles of cable. Or maybe this cable is something
> >> really exotic... always possible, given the eclectic nature of PicLister
> >> backgrounds.
> >>
> >> Reg Neale
> >
> >

1998\03\12@193157 by kirmse

flavicon
face
Here I have attempted to derive some basic numbers
for the capacitance and time constant of a cable.
I admit I am somewhat out of practice so someone
scream if I have made any major errors.

r = radius of the wire in meters
b = distance of wire to ground in meters
e = 8.854 * 10^-12 F/m


                    2 * pie * e
C/m = --------------------------
              b + (b^2 - r^2)^0.5
        ln [ -------------------- ]
                            r

I get about 8pF / meter of capacitance between a
small bare wire suspended 1 meter above ground
and ground.

The resistivity of qood quality plastic dielectrics are commonly
10^16 ohm cm and can reach 10^19 ohm cm for teflon.

For a 0.1 mm insulator and a 1mm diameter wire I get
about 0.01 cm^-1 for 1 meter of wire.

Time constants of a wire.

10^14 ohms * 8pF = 800 seconds.
10^17 ohms * 8pF > 9 days

I am making a lot of assumptions here but I could see
where you could achieve some very high time constants.

Many plastics will absorb water so these numbers can differ
by orders of magnitude.

---------------------------------------------------------------------
| Dr. Kevin Dale Kirmse
| PhD Electrical Engineer
|
| King of Prussia, PA 19406
| spamBeGonekirmsespamKILLspamnetaxs.com
---------------------------------------------------------------------

1998\03\15@160103 by Hap Wheeler

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face
I was once apprenticed to a high voltage cable splicer....(a lead wiper).
after he spliced and insulated a power cable, he would "meg" it with a
meg-ohm meter.  this was a hand held generator with a meter to measure
potential.  He would have me crank up to 10,000 volts and measure current
flow to ensure that there were no faults in the insulation.  He would
then bleed off the static charge with a hot stick with a bleeder resistor
to ensure that we would not get zapped.  As a graphic demo of how much
charge the mile or so of cable could hold, he once shorted it to ground
with a piece of rebar...it burned a bretty good chunk out of the rebar
and I never forgot to bleed off the charge after that.


HAP WHEELER
Network Manager, Suny Plattsburgh

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"Magic is real.....unless explicitly declared as integer"
                                         Wiz Zumwalt

1998\03\15@200344 by William Chops Westfield

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Don't forget that a single isolated conductor has capacitance as well.
You don't *need* two conductors.  (somewhere there's a nice formula for
the capacitance of an isolated sphere.)  A capacitor simply stores energy
in an electic field, which doesn't require two conductors (two simply
allows your field to be very concentrated...)

BillW

1998\03\16@122641 by John Shreffler

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part 0 1307 bytes
Sorry about the trailer, everyone.  Email private for latest bulletin of what I
have done to eliminate it.

{Original Message removed}

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