Re: Electrostatic Induction
- From: "Don Kelly" <dhky@xxxxxxx>
- Date: Thu, 06 Dec 2007 06:32:30 GMT
----------------------------
"Vince Morgan" <vinharAtHereoptusnet.com.au> wrote in message
news:47575fbf$0$7421$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
Perhaps this is not the right group for this enquirey but I can't find one
more suitable, and I am open to suggestions.
My technical literacy is very poor so please be patient.
Recently I took a close look at static influence machines. As you are all
aware these machines devellop HV at low current.
Originaly I was taught that these machines picked static charge out of the
air, like picking daisies. Looking at them more closely reveals that they
are using the surrounding air both as a dielectric, and as a circuit.
I wondered how the current might be increased in such machines. A few
days
ago I found the following at http://www.rastko.org.yu/rastko/delo/10881.
POSSIBILITIES OF ELECTRO-STATIC GENERATORS
by Nikola Tesla
[quote]
A remarkable device of this kind, embodying new features, has been
recently
developed by Dr. R. J. Van de Graaff at the Massachusetts Institute of
Technology, and is attracting extraordinary attention. (See page 96,
February, 1934, SCIENTIFIC AMERICAN. - Ed.) It is hailed as a
revolutionary
invention with which wonders will be achieved. The technical papers refer
to
it as a Colossus, a Master Key expected to unlock the secrets of nature.
Naturally enough imaginative scribes have built Spanish castles on this
foundation. So it comes that even such an ably edited paper as The Near
York
Times informs its readers of a contemplated use of this generator for long
distance transmission of power. According to a bona fide report in its
issue
of December 5, 1933, "the possibilities of the colossal generator have
been
worked out in theory and it now remains to apply it in practice." However
visionary this scheme may appear it is not absolutely impossible. A wise
Macedonian king said: "No wall is so high that a mule loaded with gold
could
not jump over it." With unlimited capital and regardless of returns, it
might be carried out.
In view of many articles and editorials written in the same vein, which
have
amazed the layman and amused the expert, it may not be amiss to examine
the
merits of this odd contrivance in the light of well demonstrated
scientific
facts.
Most people, and not a few electricians, will think that very long and
noisy
sparks are indicative of great energy, which is far from being the case.
An
impressive display of this kind, at several million volts, can be readily
obtained with any wide leather or fabric belt in dry weather. The only
requirement is that the outward surfaces of the highly charged capacity
elements be arranged along an ideal boundary everywhere of small
curvature.
But the electrical energy is trifling and this applies to all
electrostatic
generators which have been proposed, irrespective of size.
In estimating the amount of electricity furnished to each terminal per
second, only the sprayer need be considered as it supplies much more than
could be generated by friction of the belts. The device used has not been
clearly described but it is sufficient for the purpose of this
dissertation
to know that it operates at 20,000 volts and energizes, through rows of
points, the two belts which are said to be four feet, or 120 centimeters,
wide. Assuming that they are run at a speed of 100 feet or 3,000
centimeters
per second, the area covered in this time interval would be 120 x 3,000 =
360,000 square centimeters. If it were possible to charge the belts
uniformly to a surface density anything like that existing on an
electrified
particle, the output of the machine would be very great. But this can
never
be realized. The following approximate estimate will show what may be
reasonably expected.
[/quote]
Although he pays close attention to the charge carrier(belt) he identifies
the charge sprayer as the primary reason for the low current in the Van de
Graaff, I believe, and by extension that could be applied to all such
machines, including inluence machines like the Pidgeon or Wimhurst.
Toward the end of the document he wrote something I found very interesting
personaly.
[quote]
Static electricity may be eventually harnessed for driving motors and this
p
rospect is attractive on account of the enormous power output of such a
machine at very high voltages. The efficient generation and control of
these
is the chief impediment in this direction. As an interesting experiment
the
two units of the generator described might be separated and so an electric
drive improvised. It would be operative but inefficient.
[/quote]
If I were to locate an isolated flat metal plate, with a wire attatched
through a load to earth, such that an insulated charged body oscillating
tangentially across the surface of the plate would induce an AC current in
the wire, I could have a rather efficient electric generator? As the
charged bodies static potential (field?) is in no way diminished by the
interactions it can be viewed (without regard to leakage) as an infinite
source of potential energy. Is this true? If this is true it would imply
that one could repeatedly induce a current between potentials without
having
to continuously reestablish those pontentials. This certainly has some
philosophical implications.
One could look for manifestations of forces that would identify the work
being done to drive the circuit other than by the charged potential, but I
can't see them personaly. They must exist mustn't they? Where are they?
Many are familiar with the existence of the mysterious 'Testatica" machine
in Switzerland. Looking much like a modified Wimhurst or Pidgeon it
appears
to be capable of the impossible. However, when one examines this machine
it
become apparent that the charge collectors are a vast improvement on the
comb type usualy used in similar machines. Perhaps the designer of this
machine read Tesla's observations too.
I believe a very efficient electric generator could be devised that
incorporated the use of static fields to induce current, without
neutralising those potentials in the process.
Please feel free to offer any learned critisism of my idea. I would
appreciate it very much indeed, though philisophical arguments will be
treated as a point of view if I may.
Regards,
Vince
The problem with electrostatic machines (and such motors and generators
exist)- in addition to the necessarily high voltages is that the energy
density of such devices, at best, is far inferior to that of electromagnetic
machines. Thus a 1HP motor might require several thousand volts and be the
size of a house. Note the sizes mentioned in your reference. These are
practical considerations that limit electrostatic machines- too big, too
high a voltage and too expensive. The "sprayers" are a minor problem and are
a bit more efficient overall than friction contacts. Size is the major
problem (this has a great deal to do with the difference between mu0 and
epsilon0) and is one that can't be wished away.
The Van de Graff machine has had limited use- for a HV low current supply
for charge accelerators as an example. A small one can fill a 3 story
tower. Beyond that electrostatic precipitators are commonly used to reduce
particulates in exhaust gasses from coal fired power plants and in some
other applications. There are also electrostatic speakers, microphones and
some sensors so the use of electrostatic machines is quite common-where
their benefits overcome their disadvantages.
Where Tesla's observations are correct is in his doubt about electrostatic
machines.
As for your oscillating charged body - why do you think that this will
produce a current in the wire? --
Don Kelly dhky@xxxxxxxxxxxx
remove the X to answer
.
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