# Did Hertz discover the concept of frequency and if so, how did Doppler think of frequency?

I may not be understanding what a "Hertz" is but it seems to simply be one cycle per second. But would not Doppler have understood what frequency was decades before Hertz? Or was Hertz just the first to understand the e/m radiation had frequency just as sound does?

• Hertz is the same as cycles per second (cps), which was the unit name previously used in English-speaking countries. Karl Willy Wagner, appears to have been the first who proposed (in a 1918 publication) to name the unit of frequency after Hertz, without providing a rationale. In a committee meeting in October 1933 IEC adopted Hertz as the unit of frequency. This was confirmed when the full commission adopted the MKS system, including this unit name, in December 1935. Jan 21 at 8:25
• The "concept of frequency" was known in the ancient Greece and is actually credited to Pythagoras himself. Jan 21 at 14:27
• @AlexandreEremenko I would appreciate to see a source for that. Jan 21 at 14:28
• @AlexandreEremenko maybe you are referring to Pythagoras as in musical notes (Pythagorean scale). However we had to wait for Galileo to make the connection between musical notes and the frequency of vibration of a musical instrument. Jan 21 at 16:35
• hsm.stackexchange.com/questions/454/… Jan 22 at 0:47

Heinrich Hertz proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism.

It is neither for the discovery of frequency nor that EM had a frequency.

The concept of frequency is as ancient as the concept of time. It just indicates how many cycles there are in a given unit of time. Looking at the sun turn around Earth, astronomers in the past would have said that the frequency is 1 turn per 24 h. Even the Romans had a word for it frequentia which is the root of the English word.

Mathematically, the modern concept of waves was already fully established by the 18th century with the formulation of the wave equation.

Maxwell had predicted that light was a wave of oscillating electromagnetic fields as the equations of electromagnetism (EM) turned into a wave equation in the vacuum with the speed $$c$$ (speed of light). The idea of light being a wave was also not new (even Doppler insisted on it), as diffraction experiments in the 19th century showed it (see experiments by Thomas Young and Jean Arago).

Hertz was the one to produce the first experimental setup to create radio waves from electromagnetic principles (dipole antenna). Radio waves (RW) were also a new discovery at the time but Hertz proved that RW had the same speed and follow the same laws of refraction/reflection/diffraction as light. Light turned out to be an EM wave too but with a higher frequency than radio waves.

In 1960, during the Conférence générale des poids et mesures, the International Bureau of Weights and Measures (BIPM), made standard the use of Hertz to replace the previous unit of frequency, the "cps" (cycles per second).

Units are named after scientist in their honor but not because they discovered the units. For example, the gauss (unit of magnetic flux) is not named after Carl Friederich Gauss because he discovered magnetism. Sightings of magnetic rocks and the invention of the compass were centuries to millenia older. Gauss just did some important work on electricity and magnetism and the British Science Academy (or whoever invented it) named the unit in his honor.

Edit: Interestingly enough, with all that said, Carl Doppler avoided frequency as a variable when establishing the equations for the effect named after him. In his work Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels he writes his equation in terms of periods (time to go from point A to B).

• Doppler didn't use the word Frequenz in his paper, but he did use Schwingungszahl, that has the same meaning... Jan 21 at 16:26
• @xxavier oh my bad, my German is awful. But if I understand correctly he does not use frequency as variable but the period. Jan 21 at 16:28