How did Faraday, Ampere and Hippolyte Pixii know that electricity 'traveled' at all? Let alone in a certain direction? And that it was reversing direction with certain early generators?
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5$\begingroup$ I imagine that if you put a compass next to a wire, the compass needle feels the magnetic field from the wire. If the field is switching direction, the needle should wiggle... $\endgroup$– DanielSankCommented Oct 11, 2018 at 15:20
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$\begingroup$ You're asking two questions: your title's question and the first sentence. $\endgroup$– GeremiaCommented Oct 11, 2018 at 21:20
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$\begingroup$ They did not, they did not even know what the "current" was, it was a metaphor. There were some vague hypotheses about electric fluids with a controversy as to whether there was one or two (Ampere favored two). Galvanometer needles changing direction could distinguish AC from DC, but there was no clear understanding what it corresponded to in the wire until much later. $\endgroup$– ConifoldCommented Oct 11, 2018 at 21:46
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2$\begingroup$ Possible duplicate of What is the history of electric current and resistance? $\endgroup$– ConifoldCommented Oct 11, 2018 at 21:46
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$\begingroup$ See Faraday's Electrical generator and Hippolyte Pixii. $\endgroup$– Mauro ALLEGRANZACommented Oct 12, 2018 at 6:15
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4 Answers
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Daniel Sank is correct; with a magnetized needle on a pivot and a coil of wire you can make a device called a galvanometer with which you can watch current change direction, at least for low frequencies of reversal. The early experimenters could connect galvanometers to their electrochemical experiments and correlate the direction of needle deflection to the direction of charge transport.
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$\begingroup$ Just to be pedantic, did anyone record and publish this experiment "back then" ? $\endgroup$ Commented Oct 12, 2018 at 11:53
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Benjamin Franklin, with his lightning kite experiment, was one of the first to realize that electricity travels.
Ampère knew that parallel wires with current going the same (opposite) direction attract (repel). Thus, if there an AC, you will see the wires vibrating like a plucked string.
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Well physicists also conducted experiments on Electrochemistry and stuff like Electroplating and where one metal atom 'travels' to another may have made them think about electricity 'travelling'.
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In the 1830s many people were interested in knowing the "speed" of electricity in wires. Wheatsone designed a visual experiment to determine the speed of electricity the help of rotating mirrors. The original reference is Wheatstone, Charles (1834). "An Account of Some Experiments to Measure the Velocity of Electricity and the Duration of Electric Light". Philosophical Transactions of the Royal Society of London. 124: 583–591. Here you might get better and original references.
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$\begingroup$ Interesting but I don't think this addresses the original question -- how did they decide there was propogation of "something" in electric wires in the first place? $\endgroup$ Commented Oct 12, 2018 at 11:52
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$\begingroup$ I think it was the appearance of a spark when a current was introduced from one end and the spark appeared at another end of the wire in Wheatstone's arrangement. This gave them an idea of electric fluid is traveling in the wire. $\endgroup$– ACRCommented Oct 12, 2018 at 13:25