Heinrich Hertz and Maxwell’s theory of wave propagation

Heinrich Hertz, who demonstrated the existence of electromagnetic waves predicted by James Clerk Maxwell, died after a long illness in 1864 at the age of 36. In 1930 the International Electrotechnical Commission (IEC) established the hertz (Hz) as the unit of frequency. Along with other SI units, the Hertz replaced cycles per second in the United States as a partial move toward metrification.

By way of background, James Clerk Maxwell had posited, on theoretical grounds, the existence of electromagnetic waves. In A Dynamical Theory of the Electromagnetic Field (1865), he predicted that electromagnetic fields travel through space at the nominal speed of light. He described this electromagnetic energy as waves, a luminiferous medium being assumed to carry them just as air conveys sound waves. Maxwell believed correctly that light is a similar wave phenomenon consisting of vibrations of the same medium albeit at a different frequency.

Maxwell’s work was based on experiments performed several years earlier by Michael Faraday. However, Maxwell’s assertion that electromagnetic energy including light consisted of waves propagating through space remained strictly theoretical, based on equations that he developed. Aside from light, electromagnetic energy had not been observed to travel through space in the form of waves.

Maxwell’s work was widely accepted but as of 1885 experimental confirmation was still lacking. Finally, beginning in 1887, Hertz built apparatus that confirmed Maxwell’s theory of wave propagation. The apparatus consisted of a transmitter and receiver, with no direct electrical connection between them.

In autumn, 1886, Hertz discovered that discharging a Leyden jar into one of a pair of coils would induce an electrical spark to jump across an air gap connected to the other coil. This reaction will take place in two untuned coils in close proximity. It works at a distance when the two coils are tuned to the same frequency by means of resonant circuits.

Hertz was a modest individual and asserted that his apparatus was merely a demonstration of Maxwell’s theory of electromagnetic wave propagation, with no practical application. A few years later, Guglielmo Marconi (1874-1937) was building apparatus capable of transmitting radio signals, based on Hertz’s work, at first a distance of slightly over a mile and eventually across the Atlantic from a transmitter in Canada to a receiver in Europe. If Hertz had not succumbed to a fatal illness at an early age, it is likely that he would have lived to witness his ideas give birth to commercial radio transmission.