Re: Coherer experiments

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

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

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.

	See below.  Sophisticated coherer's were used in commercial practice
for a number of years.  Some of them were quite elaborate, and some good
descriptions are given in Zenneck's "Wireless Telegraphy", which has
photos of a lot of very complicated looking commercial receiving
equipment of the 1905 era.  The stuff is beautifully built, large and
elaborate, and probably didn't work as well as a simple crystal
detector!

> 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.

	There's a lot of peak power in that discharge!
 
> 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.

	Surely would, as there's a lot of power in the discharge, whether it be
the primary circuit or the secondary circuit.  Since the coherer is a
voltage sensitive device, suspect the secondary discharge would be the
primary actuator unless the coherer circuit was tuned to a VHF
frequency.

> Antonio Carlos M. de Queiroz

	Yes.  Although I have yet to build a coherer (it's on the agenda for
"when I have time") I have a Japanese-built radio-controlled model bus
which I acquired in 1950.  It uses a simple spark transmitter (3 D cells
powering a small buzzer-type spark coil exciting a 6" antenna) and a
receiver with a coherer.  Each time the spark is sent the coherer in the
bus "coheres", closing a sensitive relay circuit.  The relay then
supplies power to a motor which sequences through these steps:

1. Wheels run.

2. Bus turns right.

3. Bus turns left.

4. Bus runs straight.

5. Wheels stop.

At the end of each step the motor actuates a tapper which strikes the
spring-supported board on which the coherer is mounted.  The antenna on
the bus is about 12" long, and I judge the operating frequency to be of
the order of 100 MHz or more.  Of course, with this simple transmitter
the signal has a very wide spectrum.

	The range is of the order of 10 to 15 feet, and has decreased with the
years as the coherer deteriorates.  Thunder storms are rare here in
Southern California, but I have been operating the bus on a couple of
occasions when lightning strikes occured within a mile or so, and the
bus responded.

	A recent issue of the journal of the California Historical Radio
Society had a paper describing someone else's coherer experiments and I
intend to repeat them, again "when I have time".  

Ed