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A cow-orker recently "informed" me that the reason we call the speed of light $c$ is that c stands for "causality.

I would guess that can't be the case, because I think the symbol $c$ was used to represent the speed of light as early as the days of Faraday and certainly in Maxwell's papers, while the idea that light propagation and causality are related came into being decades later, with relativity.

But I'm not absolutely certain that Maxwell used the symbol $c$.

At any rate, does anyone know when using the symbol $c$ to represent the speed of light came into common practice?

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The origin can be traced back to the experiments (and subsequent papers) by Weber and Kohlrausch (1856). The paper can be found here

Although Weber constant had a magnitude around $\sqrt2c$, the same can be taken to be one of the first time the constant be denoted as 'c'.

Later (1894) Paul Drude modified the Weber's constant in such a way we use today.

Later Planck, Lorentz, Einstein etc contributed to the popularity of this notation.

To dig on the exact 'meaning', I found this:

Weber apparently meant c to stand for "constant" in his force law, but there is evidence that physicists such as Lorentz and Einstein were accustomed to a common convention that c could be used as a variable for velocity. This usage can be traced back to the classic Latin texts in which c stood for "celeritas" meaning "speed". The uncommon English word "celerity" is still used when referring to the speed of wave propagation in fluids. The same Latin root is found in more familiar words such as acceleration and even celebrity, a word used when fame comes quickly.

Source : http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html

so ... 'celeritas'

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