Hertz

Lore & Background

The hertz (Hz) is the International System of Units (SI) derived unit for frequency, defined as one periodic event or cycle per second. Its formal expression in base SI units is the reciprocal second (1/s or s⁻¹). The unit is named after German physicist Heinrich Rudolf Hertz, who provided the first conclusive proof of electromagnetic waves. It replaced the older term "cycles per second" (cps) and its multiples, such as kilocycles per second, following adoption by the International Electrotechnical Commission in 1935 and the General Conference on Weights and Measures in 1960. Common multiples include kilohertz (kHz), megahertz (MHz), gigahertz (GHz), and terahertz (THz). The hertz is used exclusively for periodic events; aperiodic or stochastic occurrences are expressed in reciprocal seconds or, for radioactivity, in becquerels. While frequency, angular velocity, and angular frequency all share the dimension of inverse time, only frequency uses the hertz. The unit applies to periodic waveforms, musical tones, radio and audio applications, and computer clock speeds. It also represents photon energy via the Planck relation, where energy equals frequency multiplied by Planck’s constant. The hertz is used to describe sound frequencies (perceived as pitch), electromagnetic radiation from radio waves to gamma rays, and gravitational wave observations.

Reader's Guide

The hertz quantifies the rate of periodic events, defined as one cycle per second. It is an SI derived unit, formally expressed as the reciprocal second (s⁻¹). The unit is named for Heinrich Rudolf Hertz, who provided the first conclusive evidence of electromagnetic waves. The International Electrotechnical Commission established the name in 1935, and it was adopted by the General Conference on Weights and Measures in 1960, replacing the earlier term "cycles per second" and its multiples like kilocycles and megacycles. The hertz is used exclusively for periodic phenomena; aperiodic events, such as radioactive decay, are measured in reciprocal seconds or becquerels. Common applications include describing sound waves, where frequency corresponds to pitch, and electromagnetic radiation, from radio frequencies in kilohertz and megahertz to visible light in terahertz and beyond. The unit also describes computer clock speeds and, via the Planck relation E = hν, links frequency directly to photon energy. In gravitational wave detection, laser interferometers operate in the 30–7000 Hz range, while pulsar timing arrays measure nanohertz frequencies. The hertz is thus essential for characterizing periodic signals across physics, engineering, and technology.

Did You Know?

• One hertz is equivalent to one periodic event per second, and the unit replaced 'cycles per second' in 1960.
• The hertz is used for periodic events only; aperiodic events use reciprocal seconds or becquerels for radioactivity.