Editing Charles Wheatstone (section)

== Velocity of electricity ==
He achieved renown by a great experiment made in 1834 – the measurement of the velocity of electricity in a wire. He cut the wire at the middle, to form a gap which a spark might leap across, and connected its ends to the poles of a [[Leyden jar]] filled with electricity. Three sparks were thus produced, one at each end of the wire, and another at the middle. He mounted a tiny mirror on the works of a watch, so that it revolved at a high velocity, and observed the reflections of his three sparks in it. The points of the wire were so arranged that if the sparks were instantaneous, their reflections would appear in one straight line; but the middle one was seen to lag behind the others, because it was an instant later. The electricity had taken a certain time to travel from the ends of the wire to the middle. This time was found by measuring the amount of lag, and comparing it with the known velocity of the mirror. Having got the time, he had only to compare that with the length of half the wire, and he could find the velocity of electricity. His results gave a calculated velocity of 288,000 miles per second, i.e. faster than what we now know to be the speed of light ({{convert|299792.458|km/s|mi/s|sigfig=3}}), but were nonetheless an interesting approximation.<ref name="Wheatstone velocity">{{cite journal |last1=Wheatstone |first1=Charles |title=An account of some experiments to measure the velocity of electricity, and the duration of electric light |journal=Proceedings of the Royal Society |date=31 December 1837 |volume=3 |pages=299–300 |doi=10.1098/rspl.1830.0178 |url=https://royalsocietypublishing.org/doi/10.1098/rspl.1830.0178 |access-date=11 March 2023}}</ref><ref name="Bowers">{{cite book |last1=Bowers |first1=Brian |chapter=The velocity of electricity |title=Sir Charles Wheatstone |date=1 January 2001 |pages=57–68 |doi=10.1049/PBHT029E_ch6 |isbn=9780852961032 |chapter-url=https://digital-library.theiet.org/content/books/10.1049/pbht029e_ch6 |access-date=11 March 2023}}</ref>

It was already appreciated by some scientists that the "velocity" of electricity was dependent on the properties of the conductor and its surroundings. [[Francis Ronalds]] had observed [[Submarine communications cable#Bandwidth problems|signal retardation]] in his buried [[Electrical telegraph#First working systems|electric telegraph]] cable (but not his airborne line) in 1816 and outlined its cause to be induction.<ref>{{Cite journal|last=Ronalds|first=B. F.|date=2016|title=Sir Francis Ronalds and the Electric Telegraph|journal=International Journal for the History of Engineering & Technology|volume=86|pages=42–55|doi=10.1080/17581206.2015.1119481|s2cid=113256632}}</ref> Wheatstone witnessed these experiments as a youth, which were apparently a stimulus for his own research in telegraphy. Decades later, after the telegraph had been commercialised, [[Michael Faraday]] described how the velocity of an electric field in a submarine wire, coated with insulator and surrounded with water, is only {{convert|144000|mi/s|km/s}}, or still less.

Wheatstone's device of the revolving mirror was afterwards employed by [[Léon Foucault]] and [[Hippolyte Fizeau]] to measure the [[Foucault's measurements of the speed of light|relative speeds of light in air versus water]], and later to measure the [[speed of light]].