Potentiometer(variable resistor/rheostat). The potentiometer is a resistor where the resistance can be changed by the operator. The potentiometer is manufactured in different shapes and values.
Most potentiometers are set by turning a shaft, a screw or a slider. You can get potentiometers where the resistance changes in a linear fashion. Others may have a logarithmic resistance change. Though potentiometers may vary in shape, the basics are the same; A resistive track with connectors in each end, the third connector is a slide that can be moved to set the resistance.

The cheapest type to make is the carbon track potentiometer. The track is made of carbon on a substrate of phenol paper. This type has a poor linearity, a poor resolution and a short lifetime. It is not suited for high effects but are easy and cheap to manufacture.

A variation on the carbon track is the conductive polymer track. Here the track is made from fine-granulated carbon mixed with a polymer. The conductive polymer track has a long life and a high resolution but has a low effect tolerance. It also has a poor temperature stability.

Cerement track: this type has a good effect tolerance, high resolution, good temperature stability, and a long lifetime. The track is made from a mix of metal and ceramics, and printed on a ceramic surface.

Wire wound track. Here the track is made from a wire that is wound round a cor of ceramic or a fibre glass. This type of potentiometer is used for high effects, and has a good temperature stability. It has poor resolution, but by coating the track surface with a conductive polymer it can be increased.

The electronic symbol: