In a nutshell, yes, yes, yes, and yes.
(more below)
Right. This is what I was looking for. I can't find info on how this
works. I dont mean details on how to do it, I mean explanation of the
principle.
The current path through the razor blade, to the graphite lead, to
the other blade, and out, has a certain resistance. And yes, the
resistance changes as the lead bounces.
So in your hint above, you run current through the razor blades, and as
the shoe box vibrates, the pencil lead bounces? It's more or less a
mechanical thing and carbon is used only because it's a conductor that
happens to bounce well?
It's a "poor" conductor, so it presents some resistance while
just sitting there. The same with the carbon grains in a Bell
phone - it's used because it's a "poor" conductor, so that
the resistance will decrease when the pressure (sound wave)
increases, pressing them together, and the resistance increases
on the rarefaction wave.
I saw a page that explains how to build a working model of Bell's first
telephone. It had a wire suspended from a diaphram over a copper cup
containing vinegar. As the diaphram vibrated, the wire would contact
the vinegar and current would be passed through the vinegar to the cup.
So that's an entirely mechanical set up.
Yup.
If I understand things, this is a little bit different though, right?
This method depends on the small amounts of current generated in the
foil as it moves w/i an magnetic field. Is that right?
Wouldn't you need some way to amplify the signal?
Yes - the carbon mic doesn't necessarily need an amp, because
it's just a variable resistance, and can be just put in series
with the speaker (which is the way old telephones work. after
all); the ribbon mic is a tiny little generator, and that signal
definitely needs to be amlpified.
Cheers!
Rich