The electromotive force is divided into motional electromotive force and induced electromotive force. Through theoretical analysis and calculation, this paper reveals that the electrostatic field loop theorem can be extended to the general electric field loop theorem: In any electric field, the line integral of the electric field intensity along any closed path is equal to zero. Maxwell's mathematical expression of Faraday's law of electromagnetic induction is incorrect. Faraday's law of electromagnetic induction should be modified to Faraday's law of open circuit electromagnetic induction. Based on Helmholtz coil, a uniform alternating sinusoidal magnetic field is formed in the central area of the coil. Two different shapes of induction coils, round and square, are used. Their sectional area and the number of coils are equal. The two induction coils with different shapes are respectively placed in the center of Helmholtz coil, and their open circuit induced electromotive force is measured. The experimental results show that the induced electromotive force is not only related to the change rate of magnetic fluxes passing through the loop, but also to the shape of the loop, so Faraday's law of open circuit electromagnetic induction not correct. This paper further reveals the metal wire law of induced electromotive force: The induced electromotive force of any metal wire is proportional to the magnetic induction intensity, angular frequency of the magnetic field wave and the projection length of the wire in the normal direction of the magnetic field.