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Re: Coherer experiments



Original poster: "Ray von Postel by way of Terry Fritz <twftesla-at-uswest-dot-net>" <vonpostel-at-prodigy-dot-net>

Antonio:
    A friend, who was trained by and worked for  the Marconi Co told me
that they usually
increased the sensitivity by placing a battery operated door bell,
(with out the bell), in such a position that the striker barely vibrated the
glass tube.  This would be a method of giving the glass tube a
"continuous"  light tap to prevent the filings from clumping up.
Your experience

    He also said, as Terry mentions in his post, the cohere used
nickel fillings.  They were considered the best detectors
available for reception of spark transmitters using
synchronous rotary gaps and arc transmitters.  Therefore, it should
work with any type signal including C.W.  One circuit even used a
small spark gap unit at the receiving end to heterodyne with the
incoming signal.

His opinion was that  the cohere was more sensitive
than the crystal set which replaced it.  The crystal set was easier to
operate and more reliable than the cohere.  The book I have, published
in 1916, shows several circuits but some are labeled as "obsolete".  A
book from 1921 just mentions the cohere and crystal sets.  By this
time the audion detectors were all the rage.

It is not surprising that Tesla used the cohere since it was either state
of the art or very close to it
during his early days.

Ray


Tesla list wrote:

> Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <acmq-at-compuland-dot-com.br>
>
> Hi:
>
> Has someone ever experimented with a "coherer", or Branly's tube?
> It's a primitive radio detector, used in the early experiments
> about radio transmission. There are many references about it in the
> web, but I didn't find any modern reproduction.
>
> I made one by enclosing some iron filings in a 3/16" plastic tube,
> loose between two 3/16" brass rods inserted in the tube. With the
> device vertical (it is to be used in the horizontal), the iron
> occupies a space of 8 mm, between the 9 mm spacing of the rods.
> Much to my surprise, this simple device "detects" a spark from the
> discharge of a Leyden jar 2 meters away, by changing resistance
> from 10 MOhms to 1 KOhm. A light tap makes it recover the high
> resistance.
>
> A 9V battery and a LED in series with the coherer results in a
> nice spark detector. Long wires increase the sensitivity.
>
> Would certainly "detect" a working Tesla coil close to it too.
>
> Antonio Carlos M. de Queiroz