The term hertz is a measure of frequency, or the speed of transmission. The frequency of electromagnetic waves generated by radio transmitters is measured in cycles per second (cps), but this designation was officially changed to hertz (Hz) in 1960. An electromagnetic wave is composed of complete cycles. The number of cycles that occur each second gives radio waves their frequency, while the peak-to-peak distance of the waveform gives the amplitude of the signal.

The frequency of standard speech is between 3000 cycles per second, or 3 kilohertz (kHz), and 4000 cycles per second, or 4 kHz. Some radio waves may have frequencies of many millions of hertz (megahertz, or MHz), and even billions of hertz (gigahertz, or GHz). Table H-1 provides the range of frequencies and their band classification. The term hertz was adopted in 1960 by an international group of scientists and engineers at the General Conference of Weights and Measures in honor of Heinrich R. Hertz (1857–1894), a German physicist.

Hertz is best known for proving the existence of electromagnetic waves, which had been predicted by British scientist James Clerk Maxwell in 1864. Hertz used a rapidly oscillating electric spark to produce UHF waves. These waves caused similar electrical oscillations in a distant wire loop. The discovery of electromagnetic waves and how they could be manipulated paved the way for the development of radio, microwave, radar, and other forms of wireless communication.

As interest in electromagnetic waves grew in the nineteenth century, a physical model to describe it was proposed. It was suggested that electromagnetic waves, including light, were like sound waves but that they propagated through some previously unknown medium called the “luminiferous ether” that filled all unoccupied space throughout the universe.

The experiments of Albert A. Michelson and Edward W. Morley in 1887 proved that the ether did not exist. Albert Einstein’s theory of relativity, proposed in 1905, eliminated the need for a light-transmitting medium, so today the term ether is used only in a historical context, as in the term Ethernet.