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Back in 1855 Weber and Kohlrausch noted that "the ratio of the absolute electromagnetic unit of charge to the absolute electrostatic unit of charge", in modern notation $\frac1{\sqrt{μ_0ε_0}}$, where $μ_0,ε_0$ are the magnetic permeability and electric permittivity of the vacuum, respectively, had the units of velocity, and determined experimentally that it was remarkably close to the speed of light. Towards the end of 1861 Maxwell derived the general formula $v=\frac1{\sqrt{με}}$ for the electromagnetic propagation speed in a medium in part III of his paper On Physical Lines of Force, and suggested, in particular, that light was a form of electromagnetic radiation, see History of Maxwell's equations. It also resolved the issue of the speed of electromagnetic propagation in a medium.

Back in 1855 (published 1856) Weber and Kohlrausch noted that "the ratio of the absolute electromagnetic unit of charge to the absolute electrostatic unit of charge", in modern notation $\frac1{\sqrt{μ_0ε_0}}$, where $μ_0,ε_0$ are the magnetic permeability and electric permittivity of the vacuum, respectively, had the units of velocity, and determined experimentally that it was remarkably close to the speed of light, see references in Electromagnetic constants and the speed of light. Towards the end of 1861 Maxwell derived the general formula $v=\frac1{\sqrt{με}}$ for the electromagnetic propagation speed in a medium in part III of his paper On Physical Lines of Force, and suggested, in particular, that light was a form of electromagnetic radiation, see History of Maxwell's equations. It also resolved the issue of the speed of electromagnetic propagation in a medium.

The observers placed at the ends of these wires, two miles apart, were provided with stop watches with which to note the moment that they felt the shock. The result of a series of careful observations was that "as far as could be distinguished the time in which the electric matter performed its circuit might have been instan taneous"". [quoted from Fahie]

The observers placed at the ends of these wires, two miles apart, were provided with stop watches with which to note the moment that they felt the shock. The result of a series of careful observations was that "as far as could be distinguished the time in which the electric matter performed its circuit might have been instantaneous"". [quoted from Fahie]

For a long time it was not only believed but even ascertained that electric signals moved not just as fast but faster than light, even "instantaneously". The original experiments involving electrostatic discharges of the Leyden jar were made even before wires were introduced. According to Fahie's History of Electric Telegraphy, one of the early experimenters, Winkler "in 1744, ascertained that the rapidity of an electric discharge was exceedingly great and comparable with the speed of lightning". He used a battery of three jars connected by an insulated wire, thirty ells long laid along the bank of the river Pleisse, whose waters formed the return half of the circuit. Lemonnier

For a long time it was not only believed but even ascertained that electric signals moved not just as fast but faster than light, even "instantaneously". The original experiments involving electrostatic discharges of the Leyden jar were made even before wires were introduced. According to Fahie's History of Electric Telegraphy, one of the early experimenters, Winkler "in 1744, ascertained that the rapidity of an electric discharge was exceedingly great and comparable with the speed of lightning". He used a battery of three jars connected by an insulated wire, laid along the bank of the river Pleisse, whose waters closed the circuit. Lemonnier made another demonstration: