Question

What are magnetic field lines?

Answer 1

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A line of force in Faraday's extended sense is synonymous with Maxwell's line of induction. According to J.J. Thomson, Faraday usually discusses lines of force as chains of polarized particles in a dielectric, yet sometimes Faraday discusses them as having an existence all their own as in stretching across a vacuum. 

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Answer 2

It is a imaginary closed curved line use to represent magnetic field.