Hertz

The hertz (symbol: Hz) is a unit of frequency derived from the International System of Units (SI) and is defined as one cycle per second. It is named after Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves.

Variations
Hertz are commonly expressed in metric prefixes (decimals):
 * A kilohertz (kHz) consists of 1,000 hertz ( 103 )
 * A megahertz (MHz) consists of 1,000 kilohertz, or 1,000,000 hertz ( 106 )
 * A gigahertz (GHz) consists of 1,000 megahertz, or 1,000,000,000 hertz ( 109 )
 * A terahertz (THz) consists of 1,000 gigahertz, or 1,000,000,000,000 hertz ( 1012 )
 * A petahertz (PHz) consists of 1,000 terahertz, or 1,000,000,000,000,000 hertz ( 1015 )
 * An exahertz (EHz) consists of 1,000 petahertz, or 1,000,000,000,000,000,000 hertz ( 1018 )

Usage
The hertz is used by the computer industry to label the frequency of electronic components, notably the clock rate of central processing units (CPU). However, as manufacturers have adopted techologies such as caching, pipelining, multi-core processing, computing efficiency can vary significantly across different processor lines. As a result, the hertz is not an ideal measurement of processing power, leading to the megahertz myth.

History in Apple products
A list of processors and clock rates of selected Apple products:
 * Apple I and II: 1Mhz MOS 6502
 * Apple III: 1.8 MHz Synertek SY6502
 * Apple II GS : 2.8 MHz WDC 65C816
 * Apple Lisa: 5 MHz Motorola 68000
 * Macintosh 128K: 7.8336 MHz Motorola 68000
 * Macintosh II: 16 MHz Motorola 68020
 * Macintosh Quadra 700: 25 MHz Motorola 68040
 * Power Macintosh 6100: 60 MHz - 66 MHz PowerPC 601
 * Power Macintosh 6400: 180 MHz - 200 MHz PowerPC 603e
 * iMac G3: 233 MHz - 700 MHz PowerPC G3
 * Power Mac G4: 350 MHz PowerPC G4 – 1.42 GHz PowerPC G4 (2x)
 * Power Mac G5: 1.6 GHz PowerPC G5 – 2.5 GHz PowerPC G5 (4 cores)
 * MacBook Pro: 1.67 GHz Core Duo (2 cores) – 2.4 GHz Core i9 (8 cores)
 * Mac Pro: 2.0 GHz Xeon 5130 (4 cores) – 2.5 GHz Xeon W-3275M (28 cores)
 * iPhone 1: 412 MHz APL0098
 * iPhone 11: 2.65 GHz Apple A13 (2 high performance + 4 high efficiency cores)