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Marc Lange writes (page 29)

It was known by about the 19th century that any action at a distance involving electric or magnetic forces would be retarded and so undermine not only spatial locality, but also temporal locality.

How was it discovered in the 19th century that electric or magnetic forces were not instantaneous?

I would be interested in survey articles, but I am mainly interested in reading how those who discovered the result described what they found.

Lange, M. (2002). An Introduction to the Philosophy of Physics Locality, Fields, Energy, and Mass. Blackwell Publishing.

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  • $\begingroup$ Does Mr. Lange provide a reference or bibliography entry for his claim? (one would hope so!) $\endgroup$
    – Carl Witthoft
    Commented May 7, 2019 at 13:13
  • $\begingroup$ @CarlWitthoft He does not provide a reference, but I don't think it is relevant to his philosophical, rather than historical, approach to fields. He explains why he focuses on electromagnetic fields rather than gravitational fields. The belief in instantaneous propagation is not strong. $\endgroup$
    – Frank Hubeny
    Commented May 7, 2019 at 15:04

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This was noticed when observations of eclipses of Jupiter satellites deviated from prediction. Before that there could be only speculations (and these speculations existed from antiquity). Jupiter satellites gave the first hard evidence.

From the very beginning, Jupiter satellites were proposed by Galileo as a natural clock for determination of longitude. For that reason, very careful observations were made, and finally the disagreement with Kepler laws was found. This happened in 17th century.

Romer and Huygens conjectured that the reason is the finite speed of light and measured it (assuming that Jupiter satellites do obey Kepler laws). They obtained a number which was not very precise but of the right order of magnitude.

Since then observations were made more and more precise.

By the way, Jupiter satellites gave the most precise way of synchronizing clocks at different locations (=measuring of longitude) until the invention of telegraph. But the method could only be applied on land, not on a ship.

https://gizmodo.com/how-the-speed-of-light-was-first-measured-1138348467

That other electromagnetic oscillations are spread with the same speed as light was discovered by Maxwell.

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  • $\begingroup$ This turns the question into "when was it realized that light was an electric and/or magnetic force"? $\endgroup$
    – Mark
    Commented May 6, 2019 at 20:27
  • $\begingroup$ @Mark: But this is well known: Maxwell made this discovery. $\endgroup$
    – Alexandre Eremenko
    Commented May 6, 2019 at 21:14
  • $\begingroup$ You should probably add that to your answer, then. $\endgroup$
    – Mark
    Commented May 6, 2019 at 21:15
  • $\begingroup$ Are there experiments that show electric attraction or magnetism travel at finite speed? (Without using light and drawing the conclusion from Maxwell by computation.) Can it be observed directly? $\endgroup$
    – Gerald Edgar
    Commented May 7, 2019 at 12:16
  • $\begingroup$ @Gerald Edgar: Nowadays, yes. Every time you measure a distance to an object with a radar, you use this finite speed. Also when you communicate with an object in the space, sufficiently remote. But of course radio waves were discovered after Maxwell, and due to Maxwell. $\endgroup$
    – Alexandre Eremenko
    Commented May 7, 2019 at 16:37

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