Millihertz

Hertz
Unit system	SI derived unit
Unit of	Frequency
Symbol	㎐
Named after	Heinrich Hertz
In SI base units	s−1

The hertz (symbol Hz) is the unit of frequency in the International System of Units (SI) and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves. Hertz are commonly expressed in multiples kilohertz (10³ Hz, symbol kHz), megahertz (10⁶ Hz, MHz), gigahertz (10⁹ Hz, GHz), and terahertz (10¹² Hz, THz). Kilo equals one thousand, mega equals one million, giga equals one billion and tera equals one trillion.

Some of the unit's most common uses are in the description of sine waves and musical tones, particularly those used in radio- and audio-related applications. It is also used to describe the speeds at which computers and other electronics are driven.

The hertz is equivalent to cycles per second, i.e., "1/second" or ${\displaystyle {\text{s}}^{-1}}$. The International Committee for Weights and Measures defined the second as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" and then adds the obvious conclusion: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, ν(hfs Cs) = 9 192 631 770 Hz."

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