Why does a carbon pile work as a rheostat? This afternoon I opened a sewing machine pedal, and inside it I found a ceramic material containing lots of small and thin black disks. I didn't expect that. I've searched on the Internet and I've found a device called carbon pile that could be the same device I found (write carbon pile disks on Google Images or try this link to see the disks).
It seems that there isn't much information about this device. The only information I've found is that the carbon pile resistance is dependent on the pressure exerted along the pile. 
So, now I know why the carbon pile is employed as a rheostat or as a potentiometer. But I don't understand why a pile made of carbon disks varies its resistance depending on the pressure. Any idea?
Perhaps this is a very basic question, but I can't think of any simple explanation.
 A: If you press a pile of carbon particles together to form a pill-like disc, you will find that the electrical resistance of the pressed disc is higher than that of a similarly-sized disc of solid carbon. This is because those particles of carbon are not all in good contact with one another. 
If you then compress that disc, you mash the particles together into better physica contact with one another, building better electrical contact between them, and the resistance of the disc drops. That resistance change can be used as a control signal.
If you mill some soft rubber into the carbon particles before pressing them into discs, then you get a disc that wants to return to its original dimensions after you release the pressure, yielding a pressure-sensitive variable resistor. 
If you instead loosely pack the carbon granules into the space between two flexible metal foil discs separated by an insulator, you can make this pressure-sensitive resistance effect so sensitive that the resulting device serves as a microphone, as used in old-style telephones and radio transmitters. 
If you form the rubber-loaded carbon into a tiny pointed cone and press its tip into another metal contact, you obtain an electrical switch whose resistance varies smoothly from infinity when the contacts are not touching to a very low value when the cone and the contact are firmly pressed together. This is the principle of the anti-induction or noiseless switch as used in audio circuits. 
