|Physical quantity (s)||frequency|
|System||International system of units|
|In SI units|
|Named after||Heinrich Hertz|
|See also: revolutions per minute|
The Hertz (with the unit symbol Hz) is the derived SI unit for frequency . It indicates the number of repetitive processes per second in a periodic signal . The unit was named in 1930 after the German physicist Heinrich Hertz .
The unit was proposed in 1930 by the "technical committee for electrical and magnetic quantities and units" of the International Electrotechnical Commission and introduced in 1935 as part of the " Giorgi unit system" or MKS unit system   , named after Heinrich Hertz. At the 11th CGPM (Conférence Générale des Poids et Mesures) in 1960, the MKS system of units was merged into the SI system of units .  Since then, the unit Hertz has replaced the unit cycles per second = cps or cps or c / s, which was previously used in English-language literature (Cycles per second).
Despite the definition, the use of the unit is not limited to periodic oscillations . Regularly recurring events, such as the frequency with which a computer creates backup copies of files or issues control commands, can also be specified in the unit Hertz (clock frequency). It is also used to scale a coordinate axis in the frequency space , for example for an absorption spectrum or a wave packet in quantum mechanics .
The Hertz unit, on the other hand, should not be used to specify the statistically averaged frequencies of random processes or to specify an angular velocity or angular frequency . Even if these sizes have the same dimension and they can therefore all be specified in the unit 1 / s, the use of different units serves to emphasize the differences between the sizes.  For radioactive decay ( activity ) the unit Becquerel is used, which is also defined as 1 / s.
Common decimal multiples
The unit Hz is often used with the following prefixes :
|Kilohertz||kHz||103 Hz||a thousand cycles per second. Very high tones are in the range of a few kHz to 20 kHz.|
|Megahertz||MHz||106 Hz||a million cycles per second. VHF broadcasters send electromagnetic waves in the range of 100 MHz.|
|Gigahertz||GHz||109 Hz||a billion cycles per second. Modern processors have a clock frequency in the range of GHz.|
|Terahertz||THz||1012 Hz||one trillion cycles per second. Visible electromagnetic waves ( light ) have frequencies in the range of 400–790 THz. See also terahertz radiation .|
|Petahertz||PHz||1015 Hz||a quadrillion cycles per second. X-rays are in this range.|
|Exahertz||EHz||1018 Hz||a trillion cycles per second. Gamma radiation is in this range.|
A selection of different phenomena of different frequencies can be found in the list List of magnitudes of the frequency .
In vibration measurement technology, the speed of a machine is given in Hertz, which is called rotational frequency . Apart from that, the unit Hertz according to DIN 1301 should not be used for the speed. Instead, the number of revolutions per minute is often given here:
- 1 Hz = 60 / min
In the case of electromagnetic waves with frequencies in the gigahertz range, the wavelength is smaller, for example: wavelength in a microwave oven around 12 cm, wavelength in satellite television reception around 2.5 cm.
In a flute or pipe, air vibrates periodically. The speed of propagation of the sound wave is around 343 meters per second ( speed of sound at an air temperature of 20 ° C). The audible tone frequencies are in the range from about 16 Hz to 20 kHz and correspond to wavelengths from a few meters to a few centimeters . The concert pitch a 1 is fixed at 440 Hz.
Consider a rope that is attached at one end and moved up and down the other. This rope swings - with a little skill - as a standing wave . The length of this wave depends on two factors, the speed of wave propagation on the rope and the frequency with which the rope is moved at the unattached end.
A wave or oscillation of any shape can be represented as a superposition of sine functions of different frequencies in a frequency spectrum , in which the amplitude is plotted as a function of the frequency. The frequency axis is usually scaled in Hertz.
- IEC History
- IEC History: 1906–1956 written by the former IEC General Secretary Louis Ruppert (PDF; 977 kB)
- Historical context of the SI
- The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e édition, 2006)". In: PTB-Mitteilungen . tape 117 , No. 2 , 2007 ( Online [PDF; 1.4) MB]).