Total primary inductance was actually determined by measuring the secondary inductance and then multiply it with the turn ratio squared. Imposed secondary voltage was then multiplied with the turn ratio, thereby yielding the primary voltage. Optional L(U(f)) dependency may be determined by regarding the fact that the mmf is proportional to the magnetizing current and thereby inversely reactance dependent at a certain fixed voltage. The voltage dependency at for instance "25Hz inductance" is therefore half that for 50Hz.