A dial is generally a flat surface, circular or rectangular, with numbers or similar markings on it, used for displaying the setting or output of a timepiece, radio, clock, watch, or measuring instrument. There are many instruments used in scientific and industrial applications that use dials with pointers as indicators of a specific physical property. Typical examples include pressure and vacuum gauges, fluid-level gauges (for fuel, engine oil, and so on), voltmeters and ammeters, thermometers and hygrometers, speedometers and tachometers, and indicators (distance amplifying instruments).
Traditionally these have been mechanical devices, but with the advent of electronic displays, analog dials are often simulated from digital measurements.
The term may also refer to a movable control knob used to change the settings of the controlled instrument, for example, to change the frequency of the radio, or the desired temperature on a thermostat.
Styles of dials:
Examples of dial usage:
Mirror dials are designed to reduce or eliminate the effect of parallax. They usually consist of a small mirrored strip running parallel to the graduations of the scale under the pointer. When the observer moves his position so that the pointer obscures the pointer's reflection in the mirror, an accurate reading may be taken.