Slide Potentiometers:

How does a slide potentiometer work?

Linear or slide potentiometers produce a resistance output that varies according to the displacement or position of a slider or wiper. They are variable resistors with three leads. Two leads connect to the ends of the resistor, so the resistance between them is fixed. The third lead connects to a slider that travels along the resistor varying the resistance between it and the other two connections. The resistance element is excited by either DC or AC voltage.

Note that "Linear" can also refer to the taper (law) defining how the output voltage varies with the motion of the slider. A linear taper means that resistance increases in direct proportion to the distance travelled along the resistive element. This contrasts with a change in resistance via a logarithmic scale (a log or audio taper).

Resistive element types can be carbon (inexpensive), cermet (relatively limited life), conductive plastic (long life), or wire wound (current drawn during use).