Hertz

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For the radio pioneer, see Heinrich Rudolf Hertz. For other uses, see Hertz (disambiguation).

The acronym "MHZ" redirects here. It may also refer to the IATA airport code for RAF Mildenhall.

The hertz (symbol: Hz) is the SI unit of frequency. Its base unit is s^-1 (also called inverse seconds, or 1/s). In English, hertz is used as both singular and plural.

One hertz simply means "one cycle per second"; 100 Hz means "one hundred cycles per second", and so on. The unit may be applied to any periodic event – for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz. The frequency of aperiodic events, such as radioactive decay, are expressed in becquerels.

To avoid confusion, periodically varying angles are typically not expressed in hertz, but rather in an appropriate angular unit such as radians per second. A disc rotating at 1 revolution per minute (RPM) can thus be said to be rotating at 0.105 rad/s or 0.017 Hz, where the latter reflects the number of complete revolutions per second.

1 History
2 SI multiples
3 Applications
4 See also

[edit] History

The hertz is named after the German physicist Heinrich Rudolf Hertz, who made important scientific contributions to electromagnetism. The name was established by the IEC in 1930 [1]. It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, replacing the previous name for the unit, cycles per second (cps), along with its related multiples, primarily kilocycles (kc) and megacycles (Mc). The term cycles per second was largely replaced by hertz by the 1970's.

[edit] SI multiples

Multiple	Name	Symbol	Multiple	Name	Symbol
10⁰	hertz	Hz
10¹	decahertz	daHz	10^–1	decihertz	dHz
10²	hectohertz	hHz	10^–2	centihertz	cHz
10³	kilohertz	kHz	10^–3	millihertz	mHz
10⁶	megahertz	MHz	10^–6	microhertz	µHz
10⁹	gigahertz	GHz	10^–9	nanohertz	nHz
10¹²	terahertz	THz	10^–12	picohertz	pHz
10¹⁵	petahertz	PHz	10^–15	femtohertz	fHz
10¹⁸	exahertz	EHz	10^–18	attohertz	aHz
10²¹	zettahertz	ZHz	10^–21	zeptohertz	zHz
10²⁴	yottahertz	YHz	10^–24	yoctohertz	yHz

[edit] Applications

Electromagnetic radiation is often described by its frequency — the number of oscillations of the perpendicular electric and magnetic fields per second — expressed in hertz.

Radio frequency radiation is usually measured in kilohertz, megahertz, or gigahertz; this is why radio dials are commonly labelled with kHz, MHz, and GHz.

Sound is a travelling wave which is an oscillation of pressure. Humans perceive frequency of sound waves as pitch. Each musical note corresponds to a particular frequency which can be scientifically measured in hertz. Although the human ear is able to perceive frequencies ranging from 16 Hz to 20,000 Hz, the average human can hear sounds between 20 Hz and 16,000 Hz.

In computing, most CPUs are labeled in terms of their clock speed expressed in megahertz or gigahertz (10⁹ hertz). The number of megahertz refers to the frequency of the CPU's master clock signal ("clock speed"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals.

Various computer buses, such as memory buses connecting the CPU and system RAM, also transfer data using clock signals operating at different frequencies in the megahertz ranges (for modern products).

Image:SI Brochure Cover.jpg

This SI unit is named after Heinrich Rudolf Hertz. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Hz). But when an SI unit is spelled out, it should always be written in lowercase (hertz), unless it begins a sentence or is the name "degree Celsius".

— Based on The International System of Units, section 5.2.