Wanting to make some negative DC to watch conventional current flow

[saturated]

Yes sir thanks I admit the part about the carbon went over my head

Maybe it's like a variable resistor? Idk

 

Seems like later they used ribbons maybe they are obsolete now too

[J M Fahey]

Yes sir thanks I admit the part about the carbon went over my head

Maybe it's like a variable resistor? Idk

Yes, it IS a sound actuated variable resistor.
Inside it has carbon which is resistive like all materials so if you apply meter probes terminal to terminal you will get a reading.
Typically between 300 and 600 ohms.
If it were a slid block, it would be fixed but it´compressed carbon dust.
Grains make POOR contact with each other , imagine grains of sand, they would not touch flat face to flat face, there is none, but at random tips so resistance is higher than in a solid block.
Did I say "poor contact"?

But if you sandwich it between 2 metal plates and PRESS them together, they make better contact and resistance lowers.
When you stop pressure resistance rises.
So yes, it is a pressure sensitive variable resistor.

If you make plates thin and flexible, Audio pressure will vary resistance.

If you apply voltage (typically 48V in old school phone lines) and put a speaker (earphone) voice coil in series, that audio modulated current will make speaker VC move and you will hear sound.

Quality is horrible, "telephone sound", but you can understand it.

Microphone and earphone can be many miles away, so TELE (far away) PHONE (sound)

Worldwide phones were based on that until the 60s or 70s and even today, if you plug an old phone into a line, it will work.

Notice there are no electronics involved, no tubes or transistors 

Our elders were incredibly clever.