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  • Author Author: jc2048
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The Gold of the Fool and the Whiskers of the Cat

jc2048

Back in February, I visited one of the small museums that exist in the town where I

live. It's a natural history museum and is based in an old church. It's a lovely

place to visit, with lots of curious things to look at. In the shop, on the way out,

there was a basket with small chunks of rocks and minerals for sale and I bought a

small piece of 'fool's gold' (iron pyrites); I figured that an old fool like me

deserved a piece of gold to treasure.

 

image

 

What I didn't know back then and have only just discovered is that iron pyrites is

a semiconductor (thank you, Wikipedia!) and in the old days was sometimes used as

the 'crystal' in a point-contact diode (for demodulating radio signals in a crystal

set).

 

Well, I thought, that deserves an experiment. So here is my attempt to make a

point-contact diode, and it turned out to be surprisingly easy. I simply broke one

of the small cubes away and held it in one clip of a 'helping hand' contraption.

 

image

 

That clip I connected to the ground of a signal generator. The drive from the

generator I fed through a 10k resistor to a piece of wire-wrap wire which I arranged

to scrape across the surface of the cube. My scope then measured the output from my

improvised diode.

 

image

 

A little movement of the wire and I soon found a spot that rectified nicely [I was just

so surprised - I really didn't think that this was going to work].

 

image

 

The forward voltage looks to be a little over a volt. That's much higher than a

silicon metal-junction diode (a Schottky diode).

 

And that's it. No need to melt silica in an oven and grow it into a crystal with

99.99-whatever % purity. Instead, an instant semiconductor straight from nature.

 

Do I get a cat badge for this foolishness?

  • in reply to jc2048

    I will work on it.

     

    I have some time now that my book is about to be released.

     

    The intriguing issue is that I can now show why permanent magnets work and still follow Maxwell's Equations.

     

    The reason the pyrite works as a semiconductor has to do with the crystal structure and how the photons flow through the wire and into the crystal.  I still need to look more deeply into what you have done, but your experiment follows my theory quite nicely.

     

    DAB

  • in reply to dougw

    I shouldn't have said it was rectifying before because it was actually set up as a diode clamp.

     

    Here it is as a rectifier with the 10k resistor as the load (yellow trace input, blue trace output). The signal generator only goes to 20V pk/pk, but it gives an idea of how it behaves with higher voltages.

     

    image

     

    To get a better idea of what it's doing, this one is with a triangular waveform. Rather than a true diode, it's more that it becomes non-linear in the reverse direction.

     

    image

     

    Finally, this is at 1MHz where it almost performs as well as my cheap waveform generator.

     

    image

     

    So, it's not much of a diode and it's very sensitive to mechanical vibration. I can now see why people who used cat's whiskers ended up spending all their time fiddling around with the things.

     

    The idea of selling fool's gold to audio cranks does appeal, though.

  • This is one of the best piece of practical experimentation I have seen in ages! What great work :-)

  • in reply to shabaz

    Here's a ZN414 radio I built soon after they launched it (the ZN414 is the part that looks like a transistor, even though it's an IC). Tuning cap was a modified trimmer. Power was a small hearing-aid battery. The earpiece jack was also the power switch. The case was clear plastic, so that I could show off my lack of soldering skills.

     

    image

  • I love this kind of off the cuff experimentation. Pretty cool. One of my scout manuals had instructions for making a crystal set ( back in the 1950s). The crystal was perhaps a piece of pyrite I am not sure. I made the cat's whisker out of a bent safety pin. The capacitor was tin foil and wax paper and the coil was enameled wire wound on a paper tube. After I wound the coil I used sandpaper to sand off an area of enamel and built a wiper to slide over the bare wire to produce a variable inductor. Thankfully there was a fairly powerful AM station in the area and the thing worked. I am not sure that it worked because it was functional or just because there was enough field density in my area to pickup that station on ones tooth fillings.

    John