Re: coherers (SSTC As a transmitter)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
> 
> Tesla list wrote:
> >
> > Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <evp-at-pacbell-dot-net>
> 
> > For filings I tried pure nickel, coin silver from a
> > pre-LBJ quarter, and filings from a current nickel.  They need to be
> > made as fine as possible, and Marconi recommended using a worn out
> > file!  I washed everything with soap and water and then proceeded to
> > file and file and file, but finally got enough of each to experiment
> > with.  The silver seemed to work best, but all of them worked.
> 
> I made some coherers with iron from nail and with stainless steel.
> Filed with a grinding wheel and sorted from dust with a magnet.
> The stainless steel filings work quite well, and don't oxidize.
> A typical coherer requires a few volts for excitation. An interesting
> fact that I could verify is that they switch to conduction in a few
> nanoseconds only.
> The coherer action is too fast for something mechanical or thermal.
> 
> Antonio Carlos M. de Queiroz

	Did you use magnetic stainless which could be separated with a magnet? 
My experience with stainless steel has been that it's very hard to file,
and I never considered it for a coherer because of that.  Have you made
any relative sensitivity measurements for various materials?  Also, what
did you use for the end plugs?  In reading the old literature I see that
materials different from the filings were used most of the time, but I'm
not sure if that really matters or not.  I've wondered if the "cohering"
is due to breakdown of very thin oxide layers on the plugs or the
filings, and with stainless steel there shouldn't be significant oxide
layers and the action would have to be due to minute air gaps in the
filings.  Have you given the matter any serious thought?

	The coherers I have built here all have exhibited essentially infinite
resistance in the OFF state, and resistance from a couple of hundred
ohms down to less than 10 ohms in the ON state.  I use an ordinary
transistor curve tracer and look at the current/voltage in only one
quadrant.  Typical "breakdown voltage" is of the order of 5 volts. 
Something I almost always observe is that the ON resistance drops to
lower and lower values as the input excitation is repeated.  For
"playing around" I just use a piezoelectric sparker in the vicinity of
the coherer which works well.  [The coherer is connected to the
measuring circuit through 1 mH series inductors in both sides.]  I also
have a small hand held spark transmitter with an 8 inch vertical
"antenna", with which I've been able to get the coherer with similar
antennas to turn ON at a distance of as much as 40 feet.  I don't know
how to estimate the voltage or power which triggers the coherer under
those conditions.

	I have a toy radio-controlled electric bus which uses a spark
transmitter similar to the one I describe above, and a coherer which is
a glass tube about 7 mm in diameter and 30 mm long, filled with a
brownish grey substance which is in the form of small round pellets of
average diameter of about a mm.  Surprisingly the relay is of the "meter
type", and the circuit operates from three 1.5 volt dry cells.  The
drive motor uses an additional four.  The toy was made in Japan in the
middle 1950's and was one of a series of radio-controlled toys sold
under the trade name of "Radicon".  When it was first given to me it had
a maximum operating range of about 30 to 40 feet, but after over 40
years that has decreased to 10 feet at best.  So far I have resisted the
temptation to take the coherer apart to try to determine the composition
of the active material.  I find the Radicon toys mentioned on the web as
"collectibles", but haven't been able to learn anything more about the
coherer and how it was made.

	It is interesting that Tesla described making "sensitive devices" using
"small round" (spherical?) particles, and wonder if the Radicon coherers
were made after reading his patents.

	I can't tell if it is the RF output of the transformer which operates
the thing, as it will also trigger on lightning strikes as much as a
mile away. I suspect that a reasonably larged TC would give a fairly
long range, even thought it doesn't have much VHF output. The RF chokes
in series with the coherer are consistent with expectation of operation
at a frequency whose wavelength is compatible with the assumption that
the 10 inch antenna is a quarter-wavelength vertical.

	Has anyone else experimented with coherers?

Ed