Based on theoretical analysis and experimental verification, this study proves that the electromotive force truly expressed by Faraday's law of electromagnetic induction is the open-circuit electromotive force of a metal loop. Faraday's law of electromagnetic induction should be revised to Faraday's law of open-circuit electromagnetic induction. The electromotive force of a closed metal loop is equal to zero, that is, the line integral of the electric field intensity along a closed metal loop must be equal to zero. Therefore, Maxwell's mathematical expression of electromagnetic induction is inconsistent with Faraday's law of electromagnetic induction. This study further proposes a symmetrical metal closed loop - an equipotential metal current ring. The charges in the equipotential metal current ring are not affected by an electric field force, and are only affected by the Lorentz magnetic field force. Theoretical and experimental verification prove that the electric potential and electric field intensity in an equipotential metal current ring are equal to zero everywhere, and a changing magnetic field cannot induce an electric field in the equipotential metal current ring. Expanding the equipotential metal current ring to the vacuum, it can be concluded that in the vacuum, a changing magnetic field cannot induce an electric field, which is a great challenge to Maxwell's "electromagnetic waves" theory.