Hertz (unit)

The hertz is the SI unit of frequency, and has units of inverse seconds. The hertz is used only for periodic phenomena. The symbol of the hertz in SI is Hz.

The hertz is named for Heinrich Hertz (1857 - 1894), an early pioneer in electromagnetic research and the first person to demonstrate the transmission of electromagnetic waves through space.

The hertz is a derived unit in the SI, equal to 1 s-1 (Hz = s-1). In pre-SI usage the unit was called "cycles," "cycles per second," or "cps," from which additional terms like "kilocycles" and "megacycles" (or "kc/s" and "mc/s") were derived.

Practical use
Household alternating electrical current is usually 50 Hz or 60 Hz. In the United States, commercial AM radio broadcasting uses frequencies between 535 kHz and 1605 kHz (1.605 MHz), and commercial FM radio broadcasting uses frequencies from 87.5 MHz to 107.7 MHz. Visible light has a frequency of about 400 THz to 750 THz.

Sounds within the range of 20 Hz to 30 kHz are audible to humans, though as people age their ability to hear the highest frequencies diminishes. Automobile engines typically idle between 10 and 20 Hz. The earth rotates with a frequency of approximately 11.57 µHz (11.57 microhertz)