The moving magnet and conductor problem is a famous thought experiment, originating in the 19th century, concerning the intersection of classical electromagnetism and special relativity. In it, the current in a conductor moving with constant velocity, v, with respect to a magnet is calculated in the frame of reference of the magnet and in the frame of reference of the conductor. The observable quantity in the experiment, the current, is the same in either case, in accordance with the basic principle of relativity, which states: "Only relative motion is observable; there is no absolute standard of rest". However, according to Maxwell's equations, the charges in the conductor experience a magnetic force in the frame of the magnet and an electric force in the frame of the conductor. The same phenomenon would seem to have two different descriptions depending on the frame of reference of the observer.
This problem, along with the Michelson-Morley experiment, formed the basis of Einstein's theory of relativity.
Read more about Moving Magnet And Conductor Problem: Introduction, Background, Transformation of Fields, Assuming Galilean Transformations, Transformation of Fields As Predicted By Maxwell's Equations, Modification of Dynamics For Consistency With Maxwell's Equations, Newton's Law of Motion in Modern Notation
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