hertz
Frequencyexact by definition
The hertz is the SI derived unit of frequency, equal to one cycle per second, so its dimension is simply s⁻¹. Its relation to the second is exact by definition. The name was adopted by the CGPM in 1960, replacing "cycles per second".
Watch out: The hertz is for cyclic events, not for arbitrary rates or for angular frequency. Angular frequency \(\omega = 2\pi f\) has units of rad/s, and writing it as Hz introduces a factor of \(2\pi\). The becquerel, also s⁻¹, is reserved for radioactive decays specifically.
| 1 Hz | 1 Hz |
The unit honours Heinrich Hertz, who in the 1880s produced and detected radio waves in the laboratory and so confirmed Maxwell's prediction that electromagnetic waves exist and travel at the speed of light.
About the hertz
Frequency and period are reciprocals, so \(f = 1/T\) and a 50 Hz mains supply completes a cycle every 20 ms. That single relation spans an extraordinary range. Human hearing runs from roughly 20 Hz to 20 kHz, concert A is 440 Hz, AM radio sits in the hundreds of kilohertz, FM in the tens of megahertz, Wi-Fi at 2.4 and 5 GHz, and visible light between about 4 × 10¹⁴ and 8 × 10¹⁴ Hz. Nothing changes in the physics across that span; only the name of the phenomenon changes.
The unit is named for Heinrich Hertz, whose experiments in the 1880s generated and detected radio waves across a laboratory bench, turning Maxwell's equations from mathematics into hardware. He reportedly saw no practical use for the effect. Within fifteen years of his death, radio telegraphy was crossing the Atlantic. The everyday trap with the unit is angular frequency: mechanical and electrical analysis usually works in \(\omega\), measured in rad/s, and \(\omega = 2\pi f\), so a 60 Hz supply is 377 rad/s. Substituting 60 where 377 belongs is off by a factor of \(2\pi\) and will not announce itself in the units, because both are formally per-second.