Hertz

The hertz (symbol Hz) is the SI unit of frequency. It is named in honor of the German physicist Heinrich Rudolf Hertz who made some important contributions to science in the field of electromagnetism.

One hertz simply means "one (event) per second"; 100 Hz means "one hundred (events) 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. The reciprocal of frequency is time (period); a frequency of 1 Hz is equivalent to a period of 1 second, a frequency of 1 MHz to a period of 1 microsecond. In older writings, e.g. pre-WWII articles about radio transmissions and its electronics, the older but similar unit cycles per second (cps) is seen, along with related multiples kilocycles, megacycles, and so forth.

SI Multiples

Name	Symbol	Powers	Hertz
1 kilohertz	kHz	10³ Hz	1 000 Hz
1 megahertz	MHz	10⁶ Hz	1 000 000 Hz
1 gigahertz	GHz	10⁹ Hz	1 000 000 000 Hz
1 terahertz	THz	10¹² Hz	1 000 000 000 000 Hz
1 petahertz	PHz	10¹⁵ Hz	1 000 000 000 000 000 Hz
1 exahertz	EHz	10¹⁸ Hz	1 000 000 000 000 000 000 Hz

Examples

50 or 60 Hz, electromagnetic — standard household AC electric power
20 Hz to ~16 kHz, acoustic — normal range of adult human hearing (most children and some animals perceive sounds outside this range)
261.626 Hz, acoustic — the musical note middle C
440 Hz, acoustic — concert pitch (A above middle C), used for tuning musical instruments
104 kHz, transitions — the clock speed of the world's first commercial microprocessor, the Intel 4004 (1971)
1–8 MHz, transitions — clock speeds of early home/personal computers (mid-1970s to mid-1980s)
30–300 MHz, electromagnetic — VHF terrestrial TV broadcasts
88–108 MHz, electromagnetic — FM radio broadcasts
3.4 GHz, transitions — clock speed of the Pentium 4 "Prescott" microprocessor (2004)
428–750 THz, electromagnetic — visible light, from red to violet
30 PHz, electromagnetic — x-rays

Hertz is also the name of the world's largest car rental company. See: The Hertz Corporation