Electric guitars have knobs on their bodies that control such functions as volume, tone, treble and bass. These controls are electrical components called potentiometers, or "pots" for short. Some guitars today are also outfitted with electric pick-up systems along with volume and tone controls. Pots generally perform a single function, but two can be mounted together concentrically to handle two tasks.
How A Pot Works
A pot is a "variable resistor." Resistive material, such as carbon, is placed in a circular shape inside the pot's case. A metal "sweeper" or "wiper arm" makes contact with the strip and is turned by a shaft with a knob on it. As the wiper arm moves across the strip, the amount of resistance between the wiper arm and either of the ends of the resistive strip changes. In the case when a pot is used in a guitar's volume control circuit, the further the pot's shaft is rotated clockwise, the louder the sound. The shaft on most pots can turn in roughly a 270-degree arc.
Pots mounted on the body of the guitar typically function as volume and tone controls. Some guitars have a tone control while others break that into two separate controls, one for treble and one for bass. Many guitar models have more than one magnetic pickup, which is a coil placed underneath the strings to generate sound. Pickups are placed at different locations on the guitar's body to give a slightly different tone. On guitars that have more than one pickup there may be more than one volume control and more than one tone control.
Concentric Guitar Pots
To save space on the body of the guitar, manufacturers may use concentric pots, where two pots are piggybacked onto each other, only requiring the space that one would normally take up. A volume and tone control, for example, can both be together, yet each can be adjusted separately. The shaft of one pot is hollow, allowing the shaft from the piggybacked pot to extend through. On the guitar's surface, a knob that has a hole in the middle is slipped over the inner shaft and affixed to the outer shaft to turn it. A separate knob is placed over the inner shaft. Each knob independently moves one of the concentric pots, thus allowing individual control of volume and tone while only taking up the space of one control.
Not So Common
Dual concentric pots are not commonly found on today's guitar bodies nor on guitar amplifiers. Where they are used most often is on inexpensive car radios and consumer products such as night stand alarm clock radios.