Coates, Peter B. Formerly, Division of Electrical Science, National Physical Laboratory, Teddington, United Kingdom.
Last reviewed:January 2020
- Analog techniques
- Digital techniques
- General-purpose frequency counters
- Measurement uncertainties
- Measurements at low frequencies
- Microwave frequency counters
- Optical frequency measurements
- Frequency standards
- Related Primary Literature
- Additional Reading
The determination of the number of cycles of a periodically varying quantity occurring in unit time. Many physical systems demonstrate cyclic behavior; that is, one or more of their properties vary in a characteristic fashion before returning to the initial value and then repeating the cycle. Examples are the angular positions of the planets and satellites in the solar system, the pressure in a cylinder in a reciprocating engine, and the heights and fields associated with surface, acoustic, and electromagnetic waves. The duration of a single cycle, the period, may vary widely, from 10−27 s for the electromagnetic field associated with a cosmic gamma ray to 108 years for the rotation of a galaxy in space. The frequency, which is the inverse of the period, is the number of cycles, including fractions, occurring in unit time. The unit of frequency is the hertz (Hz), named after Heinrich Hertz, who investigated the nature of electromagnetic radiation. Measurement of the characteristic frequencies of a system, and their variation with time or under changing conditions, yields valuable information on its properties and behavior. Together with temperature and voltage, frequency ranks as one of the quantities most often measured in modern science and technology. See also: Celestial mechanics; Cerenkov radiation; Electromagnetic radiation; Gamma-ray astronomy; Milky Way Galaxy; Wave motion
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