The cycle per second is a once-common English name for the unit of

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

now known as the

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

(Hz). The plural form was typically used, often written cycles per second, cycles/second, c.p.s., c/s, or, ambiguously, just cycles (Cy./Cyc.). The term comes from the fact that

sound wave In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

s have a frequency measurable in their number of oscillations, or '' cycles'', per second. With the organization of the

International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...

in 1960, the cycle per second was officially replaced by the

, or reciprocal second, "s⁻¹" or "1/s". Symbolically, "cycle per second" units are "cycle/second", while hertz is "Hz" or "s⁻¹". For higher frequencies, ''kilocycles'' (kc), as an abbreviation of ''kilocycles per second'' were often used on components or devices. Other higher units like ''megacycle'' (Mc) and less commonly ''kilomegacycle'' (kMc) were used before 1960 and in some later documents. These have modern equivalents such as kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). Following the introduction of the SI standard, use of these terms began to fall off in favor of the new unit, with hertz becoming the dominant convention in both academic and colloquial speech by the 1970s. The rate at which

aperiodic A periodic function is a function that repeats its values at regular intervals. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to desc ...

or stochastic events occur may be expressed in becquerels (as in the case of

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

), not hertz, since although the two are mathematically similar, by convention hertz implies regularity where becquerels implies the requirement of a time averaging operation. Thus, one becquerel is one event per second ''on average'', whereas one hertz is one event per second on a regular cycle. Cycle can also be a unit for measuring usage of reciprocating machines, especially presses, in which cases ''cycle'' refers to one complete revolution of the mechanism being measured (i.e. the shaft of a

reciprocating engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fe ...

). Derived units include cycles per day (cpd) and cycles per year (cpy).

References

{{reflist Units of frequency

See also

References