Why EMF of self-induction and voltage across the coil have opposite signs?

Consider a coil whose resistance is negligible. Since the resistance of the coil is zero, the tension inside the conductor must also be zero, otherwise the current through the coil would be infinitely large. Therefore, the intensity of the vortex field created by the alternating magnetic field must be equal and opposite in the direction of the intensity of the electrostatic field created by the charges located at the terminals of the source. The potential difference is directly proportional to the strength. Consequently, the EMF of self-induction and the voltage across the coil have opposite signs.