Rheostat dials are a component of rheostats, a type of two-terminal variable resistormade up of one end contact and a wiper. The dials are used to display the degree of change in resistance that is made when a user turns the rheostat knob.
Rheostats are a common way to vary the resistance in a circuit. They are now often referred to by the more general term potentiometer. A user turns a rheostat knob to point to a given number on the dial, which alters the resistance of the device. Higher power rheostats use a wire wound around an insulator, and the resistance changes depending on where on the wire the wiper is positioned.
Rheostat dials differ depending on their function. Some are simply a kind of flat panel display engraved with numbers, while others have a protruding dial with numbers on it, designed to be used with a shaft.