${\displaystyle U_{\text{i}}={\displaystyle \underset{r_{\text{a}}\left(\text{C}\right)}{\overset{r_{\text{b}}}{\int }}\left(-\frac{\partial A}{\partial t}+v\times B\right)\cdot \text{d}r}}$

where A and B are respectively a magnetic vector potential and the magnetic flux density at a point of the path C, v is the velocity with which that point is moving, r is position vector of the point, and t is time

Note 1 to entry: If the points a and b are at rest, i.e. their velocities are zero (v_a = v_b = 0), the induced voltage is equal to the time derivative of the protoflux corresponding to the path C, with a positive or negative sign according to the convention in IEC 60375.

Note 2 to entry: The first term in the integrand results from Faraday’s law (see Maxwell equations) and the second one from the non-relativistic Lorentz transformation of the electromagnetic field tensor.