That radio was more than likely what is called a crystal
set.  In this case, crystal doesn't mean a piece of quartz to
keep a steady frequency, but means a crystal of something like
lead ore also known as galena, Carborundum and even the edge of
a rusty raiser blade.

	There was also a liquid detector made of nitric acid and
wire.  All these substances form diodes which can rectify an RF
signal picked up on an antenna and fed through a tuning network
and a set of high-impedance headphones such as 2,000-ohm
headphones.

	In the crystal sets that used the galena crystals, one
had to mount the galena sample in a plug of lead and then probe
the exposed part of the crystal with a sharp wire known as a cat
whisker until you found an active region, one that had the right
kind of impurities in it to make it rectify.  This was a PN
junction diode provided to us by Mother Nature.

	You can take the very same setup and substitute a modern
germanium or silicon diode and get much better results.

	The few millivolts of RF from a strong AM radio station
are quite enough to produce audio in the headphones.

	Not trying to belittle the technology discussed in the
article, but the laws of physics haven't changed.  You certainly
can retrieve electric power out of the air from nearby radio
stations and cell phone towers.  The trick is how much power do
you get?

	That's where the little wall-mounted transmitter they
mention comes in.  It probably transmits microwaves to the
device which has a good antenna at that frequency and lets it
recover a few volts of DC.  If you just tried to pick power out
of the air from local radio sources, you will get precious
little energy.

	As the article states, today's electronics can run on
very little energy.  If you occasionally need bursts of higher
power, your device could charge a big filter-type capacitor and
dump brief high-current pulses to something.  It might be
another day or two before the cap is charged again, but it is a
plausible method for getting power for seemingly nothing.

	Articles like this are good food for thought when
discussing electronics and power management.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Network Operations Group

tillfam3 writes:
> Thanks Howard for the great article.  It reminds me of a hamfest I went to
> and the gentleman was displaying a set of radios that he made himself that
> received a AM or a FM station without using batteries or any other power
> what so ever.  You had to listen by using a headset, but the radio station
> came in well.  Sounds like the possibilities can be endless.