The hertz (symbol: Hz) is a measure of frequency, informally defined as the number of events occurring per second. It is the basic unit of frequency in the International System of Units (SI), and is used worldwide in both general-purpose and scientific contexts. Hertz can be used to measure any periodic event; the most common use for hertz is to describe frequency of rotation, in which case a speed of 1 Hz is equal to one cycle per second.

More formally, the unit hertz is defined by the International System of Units (SI) such that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, \nu (hfs Cs) = 9 192 631 770 Hz.. Equivalently, 1 Hz = frac: 9,192,631,770 \nu (hfs Cs). This definition is derived from the SI definition of the second. The base unit for hertz is s^-1 (also called inverse seconds, reciprocal seconds). As hertz is commonly used to measure rotational speed, the base unit is sometimes represented as cycle/s.

In English, hertz is used as both singular and plural. As any SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 10³ Hz), MHz (megahertz, 10⁶ Hz), GHz (gigahertz, 10⁹ Hz) and THz (terahertz, 10¹² Hz). One hertz simply means one cycle per second (typically that which is being counted is a complete cycle); 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" (activity) of aperiodic or stochastic events, especially radioactive decay, is 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 60 revolutions per minute (RPM) can thus be said to be rotating at ≈6.283 rad/s or 1 Hz, where the latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in Hertz and an angular frequency ω measured in radians/s are:

\omega = 2\pi f and f = \omega/(2\pi) \, .

History

The hertz is named after the German physicist Heinrich Hertz, who made important scientific contributions to electromagnetism. The name was established by the International Electrotechnical Commission (IEC) in 1930. 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.''