Therefore, the differential term Vsen ??? b???Vsen ??? a is a function of only the ratio ?? 2 - ?? 1 ?? 2 + ?? 1 , [0125] which varies in accordance with the magnetic flux (or magnetic flux density or magnetic field intensity) in the transformer cores 20 a and 20 b. [0126] As compared to Eq. (17), discussed above and in which the voltage Vsen is a function of the ratio ?? 1 ?? 2 , [0127] the diffe