Re: Hertz`s experiment (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 05 Feb 2004 08:24:37 -0800
From: Ed Phillips <evp@xxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Hertz`s experiment (fwd)

High Voltage list wrote:
>
> Original poster: Steven Roys <sroys@xxxxxxxx>
>
> ---------- Forwarded message ----------
> Date: Wed, 04 Feb 2004 21:24:19 +0100
> From: Finn Hammer <f-h@xxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Hertz`s experiment
>
> I would like to try repeating Heinrich Hertz experiment from 1888, where
> he demonstrated the existance of the electromagnetic waves, previously
> described by James C. Maxwell.
>
> I haven`t managed to find a lot about the actual experiment, below is
> the best that`s come up so far.
> Anyone know where I can get the full story?
>
> http://people.deas.harvard.edu/~jones/cscie129/lectures/lecture6/hertz/Hertz_exp.html
>
> http://earlyradiohistory.us/1901hz.htm
>
> Cheers, Finn Hammer

	About 20 years ago I more or less duplicated some of his experiments,
including reflection of signals; the wavelength was just below a meter.
The apparatus consisted of a 1/2" spark coil for source of power and a
dipole antenna made up of a pair of 12" long copper wires with a spark
gap made of 1/4" diameter balls across the center gap.  Although I fed
the antenna through RF chokes that's probably unnecessary.  The place I
differed was in the receiver.  Instead of using his resonant loop and
micro spark gap, I used another dipole with a crystal detector feeding a
microammeter.  With this setup I could get a usable meter deflection to
a distance of around 10 feet, or several wavelengths.  When I
substituted an audio amplifier and headphones for the meter I could hear
signals out to maybe 40 feet.

	With this setup I could do such things as set the "receiver" on a box
and move a large metal plate back and forth behind it and observe the
rise and fall of the meter reading due to interference between the
direct and reflected waves.  This also permitted an estimate of the
wavelength.  Note that these simple transmitters had a very wide
bandwidth so that sharp nulls are not obtainable.

	I should mention that a lot of the early work involved a dipole antenna
with big sheets of metal at the ends.  That is unnecessary and Hertz
abandoned the idea in some of his later work around 600 MHz.

	I think all of the stuff has been dissambled and used for something
else now but I could reconstruct a few more details from memory if
desired.  The transmitter part is easy enough.  I had the antenna
mounted horizontally about two feet above a box containing the spark
coil and bettery to run it and connected by long spirals of wire forming
the RF chokes.   The receiver had a small RF choke across the middle gap
to provide a DC return for the rectifier, which was a germanium diode.
The output of the diode went to a small capacitor with the meter
directly in parallel, and the return was to the other end of the
RF choke.  I think that's all the detail you need to throw together
something to start experiments, and that a couple of hours work at most
will give you something of interest.

	I have a lot of literature here with much more detail of Hert's work,
including a reprint of his original paper (at least an English
translation thereof) in the IRE Proceedings celebrating the 50th
anniversary of his original work.  I think a detailed web search should
bring you more references than you have time to read.

	On the subject of repeating early experiments I'd like to see someone
duplicate the work that Bose did in India.  For those not familiar with
it he set up an ~ 50 GHz (!!!!) transmitter using a small spark in a
waveguide with a horn antenna on the end.  He had a number of different
detectors, at least some of which were clearly point-contact
rectifiers.  All this circa 1897!  He did experiments in reflection,
polarization, etc.  There are very good details of this work on the web
including photographs of a lot of his original equipment, which still
survives.  If anyone is interested I can post the URL, which isn't on
the machine I'm using now.

	Fun subject for those interested in delving into the past, and not much
work to get started.

Ed