Richard Harrison wrote:


A Faraday screen, shield, or cage is a network of parallel wires or
strips connected together at one end but disconnected from each other
at their opposite ends. I`ve worked at several broadcast stations which
used Faraday screens

The special comb-like structure that Richard describes, which is
deliberately constructed to block electric fields but transmit magnetic
fields, is normally called a Faraday "screen" - but not a cage.

The term Faraday "cage" is reserved for a complete conducting enclosure
that blocks both electric and magnetic fields from entering the
interior.

The rest of the discussion is about how well an incomplete or penetrated
enclosure might work as a Faraday cage.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:
"---but not a cage."

A cage according to my American dictionary is:
"A boxlike receptacle or enclosure for confining birds or other animals,
made with openwork of wires, bars, etc."

Ian sent me to my dictionary of electronics which reads:
"Faraday cage-See Faraday Shield"

Usage varies from place to place. I don`t know if I`m vindicated or
stand corrected.

Best regards, Richard Harrison, KB5WZI

In message , Richard
Harrison writes
Ian White, G3SEK wrote:
"---but not a cage."

A cage according to my American dictionary is:
"A boxlike receptacle or enclosure for confining birds or other animals,
made with openwork of wires, bars, etc."

Ian sent me to my dictionary of electronics which reads:
"Faraday cage-See Faraday Shield"

Usage varies from place to place. I don`t know if I`m vindicated or
stand corrected.

Best regards, Richard Harrison, KB5WZI


Was it not an Ice Bucket which Faraday used to demonstrate the fact that
the electrostatic charges repelled each other as far as possible, and
therefore stayed on the outside of the bucket? The inside was
electrically dead.
Ian.

--

On Tue, 28 Dec 2004 21:46:06 +0000, Ian Jackson
wrote:

Was it not an Ice Bucket which Faraday used to demonstrate the fact that
the electrostatic charges repelled each other as far as possible, and
therefore stayed on the outside of the bucket? The inside was
electrically dead.

Hi Ian,

Maybe it was part of an office party. Anyway, Gauss demonstrated that
electric charge FIELD LINES prefer as much separation as possible
(which conforms to your charges being repelled). With a curvature,
the field line normal to the surface will either cause line crowding
or line spreading depending upon the geometry. With a positive
curvature (the outside of a conducting shell) the lines spread; with a
negative curvature (the inside of a conducting shell) the lines
converge. Given that the bucket is conductive inside and out, he
demonstrated that line proximity within the bucket drove the charges
outside. This is not quite an issue of charges being repelled as far
as possible, or they would be uniformly distributed inside and out.

By the same logic (and experience), charge will accumulate on the
surface at the smallest radius - hence the points on lightning rods.
By extension, this is also the source of capacitor failure at either
the edges (smallest radius of a plate) or in surface burrs.

HCJB, in Quito, suffered from corona discharge and converted to loops
(misnomer, actually box), they still suffered when the corners
(smallest radius) supported the same discharge (being corner fed).
They shifted to a center feed point and put the hi voltage nodes at
the middle of a wire span.

73's
Richard Clark, KB7QHC

In message , Richard Clark
writes
By the same logic (and experience), charge will accumulate on the
surface at the smallest radius - hence the points on lightning rods.
By extension, this is also the source of capacitor failure at either
the edges (smallest radius of a plate) or in surface burrs.


Again scraping the very bottom of the memory banks, I seem to recall
that when lightning rods were first used (in the late 1700s), the
British used sharp points. The French, in the spirit of one-upmanship,
decided that theirs should have brass balls. DOH!!!
Ian.
--

"Ian Jackson" wrote
, Richard Clark writes
By the same logic (and experience), charge will accumulate on the
surface at the smallest radius - hence the points on lightning rods.
By extension, this is also the source of capacitor failure at either
the edges (smallest radius of a plate) or in surface burrs.


Again scraping the very bottom of the memory banks, I seem to recall
that when lightning rods were first used (in the late 1700s), the
British used sharp points. The French, in the spirit of one-upmanship,
decided that theirs should have brass balls. DOH!!!
Ian.
--

Very interesting! However the American Benjamin Franklin's pointed
lightning rods (it was not a British design) was never scientifically
challenged until a couple of years ago. Scientists have now shown that
blunt-tipped air terminals are attached by lightning with significantly
higher frequency than sharp tipped rods are. Pretty amazing that it took
over 230 years to "discover" this! So scrap the concept that a sharp edge
attracts charges, at least it does not attract lighting, the ultimate
charge.

http://www.usatoday.com/weather/reso...-rod-tests.htm
http://www.esdjournal.com/articles/f...n/franklin.htm
http://www.mikeholt.com/news/archive...tningblunt.htm
etc, etc

Jack Painter
Virginia Beach VA

There is more to the Franklin rods used in England: George III is said to
have required the ends to be converted to round from pointed when the
Revolution started - a pointed slam at Dr. Franklin. Nevertheless, the
houses (once there were two) of parliament were protected by Dr. Franklin's
rods.

It would have been so easy for the English to have co-opted Dr. Franklin
and quite changed the course of history. Instead, he conned the French out
of the critical support needed to win our freedom. 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:



Very interesting! However the American Benjamin Franklin's pointed
lightning rods (it was not a British design) was never scientifically
challenged until a couple of years ago. Scientists have now shown that
blunt-tipped air terminals are attached by lightning with significantly
higher frequency than sharp tipped rods are. Pretty amazing that it took
over 230 years to "discover" this! So scrap the concept that a sharp edge
attracts charges, at least it does not attract lighting, the ultimate
charge.


http://www.usatoday.com/weather/reso...ghtn-rod-tests
..htm
http://www.esdjournal.com/articles/f...n/franklin.htm
http://www.mikeholt.com/news/archive...tningblunt.htm
etc, etc

Jack Painter
Virginia Beach VA

On Tue, 28 Dec 2004 18:56:13 -0500, "Jack Painter"
wrote:


"Ian Jackson" wrote
, Richard Clark writes
By the same logic (and experience), charge will accumulate on the
surface at the smallest radius - hence the points on lightning rods.
By extension, this is also the source of capacitor failure at either
the edges (smallest radius of a plate) or in surface burrs.


Again scraping the very bottom of the memory banks, I seem to recall
that when lightning rods were first used (in the late 1700s), the
British used sharp points. The French, in the spirit of one-upmanship,
decided that theirs should have brass balls. DOH!!!
Ian.
--

Very interesting! However the American Benjamin Franklin's pointed
lightning rods (it was not a British design) was never scientifically
challenged until a couple of years ago. Scientists have now shown that
blunt-tipped air terminals are attached by lightning with significantly
higher frequency than sharp tipped rods are. Pretty amazing that it took
over 230 years to "discover" this! So scrap the concept that a sharp edge
attracts charges, at least it does not attract lighting, the ultimate
charge.

http://www.usatoday.com/weather/reso...-rod-tests.htm
http://www.esdjournal.com/articles/f...n/franklin.htm
http://www.mikeholt.com/news/archive...tningblunt.htm
etc, etc

Jack Painter
Virginia Beach VA


Jack,

All three references are of the same article. Note the rebuttals at
the end of one of them.

I would also find it hard to believe that ANY rods on a 12000 foot
mountain were not hit in 7 years!

That study would suggest that pointed rods were excellent lightning
repellers and would protect things from being struck. Exactly what
Franklin first thought.

If not excellent repellers then it would be highly suspect of the
placement of the pointed rods on the mountain.

73
Gary K4FMX

Jack Painter wrote:
"Scientists have now shown that blunt-tipped air terminals are attached
by lightning significantly higher frequency than sharp rods are."

I would have expected that sharp-pointed rods would be struck more
often.

My CRC "Handbook of Chemistry and Physics" starts its coverage of
"Electricity and Magneyism with a page on Spark Gap Voltages. In every
case for a given breakdown voltage, the gap must be substantially wider
when the electrodes are needle-points than when they are spheres. For
example: With a voltage across the electrodes of 5 KV, the gap space
between needle-points needed to prevent a spark is 0.42 cm. The gap
between 5-cm sphheres is 0.15 cm under the same conditions. Much closer
before sparking points obviously means sharp points engourage breakdown
of the air between the points, while spherical (blunt) spark-gap
electrodes discourage the spark.

It`s been said that if the chsrges dont pile op at the pointed end of a
conductor, it would not have an equipotential surface as is required by
the conductivity ("College Physics" by Franklin Miller, Jr.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian White, G3SEK wrote:
"---but not a cage."

A cage according to my American dictionary is:
"A boxlike receptacle or enclosure for confining birds or other animals,
made with openwork of wires, bars, etc."

Ian sent me to my dictionary of electronics which reads:
"Faraday cage-See Faraday Shield"

Usage varies from place to place. I don`t know if I`m vindicated or
stand corrected.

Me neither! The main lesson is that we have to be careful to define what
we mean, because there's a strong risk that other people might
understand something different.

Faraday cages are used at CERN and other large particle accelerators, to
keep the sensitive particle detectors isolated from the pulsed megawatts
of RF energy that are kicking the particles around the ring. CERN is an
international facility, so each country has its own experiments using
separate Faraday cages.

Several years ago, I needed to call a friend who was working at CERN.
Someone picked up the phone, and a voice said "British Cage".

"Well," I thought, "that certainly puts us in our place..."


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek