Hertz

The hertz is defined as one per second for periodic events. The International Committee for Weights and Measures defined the second as "the duration of 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: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly , ." The dimension of the unit hertz is 1/time (T−1). Expressed in base SI units, the unit is the reciprocal second (1/s). In English, "hertz" is also used as the plural form. As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, ), MHz (megahertz, ), GHz (gigahertz, ) and THz (terahertz, ). One hertz (i.e. one per second) simply means "one periodic event occurs per second" (where the event being counted may be a complete cycle); means "one hundred periodic events occur per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at , or a human heart might be said to beat at . The occurrence rate of aperiodic or stochastic events is expressed in reciprocal second or inverse second (1/s or s−1) in general or, in the specific case of radioactivity, in becquerels. Whereas (one per second) specifically refers to one cycle (or periodic event) per second, (also one per second) specifically refers to one radionuclide event per second on average. Even though frequency, angular velocity, angular frequency and radioactivity all have the dimension T−1, of these only frequency is expressed using the unit hertz. Thus a disc rotating at 60 revolutions per minute (rpm) is said to have an angular velocity of 2 rad/s and a frequency of rotation of . The correspondence between a frequency f with the unit hertz and an angular velocity ω with the unit radians per second is :\omega = 2\pi f and f = \frac{\omega}{2\pi} . ==History==

The hertz is named after the German physicist Heinrich Hertz (1857–1894), who made important scientific contributions to the study of electromagnetism. The name was established by the International Electrotechnical Commission (IEC) in 1935. It was adopted by the General Conference on Weights and Measures (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 per second" (kc/s) and "megacycles per second" (Mc/s), and occasionally "kilomegacycles per second" (kMc/s). The term "cycles per second" was largely replaced by "hertz" by the 1970s. ==Applications==

with varying frequency is an example of a non-sinusoidal periodic phenomenon that may be analyzed in terms of frequency. Two cycles are illustrated. Sound and vibration Sound is a traveling longitudinal wave, which is an oscillation of pressure. Humans perceive the frequency of a sound as its pitch. Each musical note corresponds to a particular frequency. An infant's ear is able to perceive frequencies ranging from to ; the average adult human can hear sounds between and . The range of ultrasound, infrasound and other physical vibrations such as molecular and atomic vibrations extends from a few femtohertz into the terahertz range and beyond. For personal computers, CPU clock speeds have ranged from approximately in the late 1970s (Atari, Commodore, Apple computers) to up to in IBM Power microprocessors. Various computer buses, such as the front-side bus connecting the CPU and northbridge, also operate at various frequencies in the megahertz range. == SI multiples ==

Higher frequencies than the International System of Units provides prefixes for are believed to occur naturally in the frequencies of the quantum-mechanical vibrations of massive particles, although these are not directly observable and must be inferred through other phenomena. By convention, these are typically not expressed in hertz, but in terms of the equivalent energy, which is proportional to the frequency by the factor of the Planck constant. == Unicode==

The CJK Compatibility block in Unicode contains characters for common SI units for frequency. These are intended for compatibility with East Asian character encodings, and not for use in new documents (which would be expected to use Latin letters, e.g. "MHz"). • (, ) • (Hz) • (kHz) • (MHz) • (GHz) • (THz) == See also ==