Hertz

Hertz. The hertz is the unit of frequency in the International System of Units, defined as one cycle per second. It is named for the German physicist Heinrich Hertz, who made pivotal contributions to the study of electromagnetic radiation. The unit is used to measure periodic events across science, technology, and daily life, from the oscillation of a pendulum to the clock speed of a central processing unit.

Definition and symbol

The hertz is defined as the number of occurrences of a repeating event per one second, making it a derived unit expressed in terms of the SI base unit for time. Its symbol is Hz, which is always capitalized because it is derived from a proper name, following the conventions established by the International Bureau of Weights and Measures. One hertz signifies exactly one cycle per second, a definition formalized by the General Conference on Weights and Measures. This precise measurement is fundamental in fields like metrology and quantum mechanics, where exact timing of oscillations is critical.

SI multiples

Due to the vast range of frequencies encountered, standard SI prefixes are commonly used to denote multiples and submultiples of the hertz. Common high-frequency multiples include the kilohertz (kHz, 10³ Hz), megahertz (MHz, 10⁶ Hz), and gigahertz (GHz, 10⁹ Hz), which are frequently used in describing radio wave transmissions and computer processor speeds. For extremely high frequencies, such as those of gamma rays, units like the terahertz (THz, 10¹² Hz) are employed. Conversely, for very low frequencies like those of geophysical phenomena, submultiples like the millihertz (mHz, 10⁻³ Hz) are applicable.

History

The unit's name was adopted by the International Electrotechnical Commission in 1930, replacing the earlier term "cycles per second." This change honored Heinrich Hertz, who in the late 1880s conclusively proved the existence of electromagnetic waves predicted by James Clerk Maxwell. His pioneering experiments with spark-gap transmitters and radio wave detection laid the groundwork for modern wireless communication. The formal adoption of the hertz into the SI system was later confirmed by the General Conference on Weights and Measures in 1960, cementing its role in global scientific discourse.

Applications

The hertz is a fundamental measurement in countless technological and scientific domains. In telecommunications, it describes the carrier wave frequencies used by AM and FM broadcasting, television, and cellular networks. Within computing, the clock speed of a microprocessor is measured in hertz, directly impacting performance. In acoustics, the pitch of a sound is determined by its frequency, with human hearing typically ranging from 20 Hz to 20 kHz. Furthermore, the hertz is crucial in physics for describing the frequency of light in the electromagnetic spectrum and the resonant frequencies of atomic clocks, which define the second.

Related units

Several other units are closely related to the hertz, often describing rotational or angular frequency. The radian per second (rad/s) is the SI derived unit for angular frequency, used in contexts like rotational mechanics and signal processing. In fields like astronomy and electrical engineering, the revolutions per minute (rpm) is a common unit for rotational speed. While not an SI unit, the baud rate in data transmission is sometimes incorrectly conflated with hertz; baud measures symbol rate, not frequency. The historical unit "cycles per second" (cps) is now obsolete but was functionally identical to the hertz.