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G0TEZ > TECH 23.03.10 08:12l 99 Lines 4293 Bytes #999 (0) @ WW
BID : 37667-GB7FCR
Read: GUEST
Subj: Re: THE LEYDEN JAR
Path: IZ3LSV<IK2XDE<DB0RES<DK0WUE<GB7FCR
Sent: 100323/0028Z @:GB7FCR.#16.GBR.EU #:37667 [Blackpool] FBB-7.03a $:37667-GB
From: G0TEZ@GB7FCR.#16.GBR.EU
To : TECH@WW
I found the history of the Leyden jar particularly interesting as put
forward by Ozwaldo LW1DSE.
When I was a boy I read about a demonstration which Michael Faraday 1791 -
1867 used to use to demonstrate capacitance to his students.
He would take a glass tumbler and wrap a layer of what we would now call
'foil' around the outside and line the inside with a similar layer of
foil.
He would then charge up this capacitor with a Wimshurst machine and, using
an insulated pair of tongs, would almost connect the inner to the outer,
producing a fat spark.
He then recharged the device and dismantled it, placing all three
components in different drawers for days' weeks, even months after which
he would reassemble the device and, once again, produce a spark.
His question to his students was "where did the electricity go ?" The
answer which he usually had to give was that it was stored in the glass by
causing 'Dielectric Stress'.
We all know about Faraday because there is a unit named after him i.e.
"When one coulomb of electricity cases a potential difference of one Volt
across a capacitor then the capacitance is 1 Farad".
None of the above is taken from the 'net, especially not from Wikipedia.
Just my long ago schooldays.
If no one is bored and, who cares ? here is the story of one of my
inventions from the late 1960s:
Video recording back then involved wide tapes with high tape speeds. The
helical drum VCRs came in the mid 70s.
So I had an idea. Thinking of my schoolboy physics and Faraday's
demonstration I built an electrostatic, slow speed tape recorder as the
problem with magnetic tape was due to the difficulty in creating a narrow
head gap.
I used plastic tape. This was easy as used 8mm film which was exactly the
same size as the magnetic tape used in tape recorders was easy to obtain.
I took a scrap audio reel to reel tape recorder and removed the magnetic
heads. In their place I put a flat steel plate, insulated from the body
and
clamped in the end of a piece of welding rod, I placed an ordinary razor
blade. I connected the plate to the chassis of an all valve TV set and the
blade I connected to the anode of the video o/p valve. I then recorded the
content of the TV picture.
For playback, I just connected the blade to the gride of the video o/p
valve and was successful in viewing a picture. It was not as easy as it
sounds. There were problems with stray capacitance and inductance which I
overcame by using stiff, copper wire, well away from the chassis and other
components. Getting the edge of the razor blade to almost but not quite
touch the moving tape was also a problem.
There was a plentiful supply of test cards back then which indicated the
resolution of the pic by blocks of fine lines marked from 1MHz to 5.11
MHz,
the maximum resolution available on a good TV on the 625 line system.
The best |I ever managed to obtain was 2MHz resolution which came close to
the 3.5MHz of the 405 line system and the quality of the VHS recorders of
12 years later.
There were two reasons why I never patented it. 1. I believed that
magnetic video recording would be overtaken in a couple of years by simply
storing a picture in RAM. This never happened because of price with the
exception of a pre production prototype made by Akai in 1972, a 12" TV
with a flat screen. Akai included an option of recording up to half an
hour of mono picture to RAM as an additional option.
2. The price of a patent then was UKP 20 (about a week's wage for me)
which would protect the idea for one year when it went up to UKP 100 for
the second year.
The point of electrostatic recording was, of course, that a razor blade
can be made far thinnner than a magnetic gap but, I always believed in
Dielectric Stress just like Faraday and, before I get a lecture, yes, of
course I know about air spaced capacitors and even, electrolytics. If the
dielectric has no efect on storing a charge, why do we have electrolytics
?
Most definaitely an interesting bull, Ozwaldo - thank you.
I am not expecting any replies as no one seems to care anymore but...
73 - Ian, G0TEZ
Msg timed: 00:26 GMT on 2010-Mar-23
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