Hertz

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&#160;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&#160;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.

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. 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.''

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).