Coherer experiments

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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