Among the relevant work that helped establish this fact : the Michelson-Morley experiment, which you've undoubtedly read about; and the experiments that established Vacuum permittivity and Vacuum permeability constants.
2.
The permeability constant ( " ?" 0 ), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum.
3.
It can be derived mathematically in terms of the permeability constant ( \ mu _ 0 ), the radius of the rails ( which are assumed to be circular in cross section ) ( r ), the distance between the central axes of the rails ( d ) and the current ( I ) as follows:
4.
Where, \ phi _ B is the flux of magnetic field, \ mu _ 0 is permeability constant 4 \ pi \, 10 ^ {-7 } \, \ textrm { Wb / A . m }, I _ c is the coil current, N is the number of coil turns per unit length, and r _ 0 is the mean radius of the coil turns.
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