Geomagnetically induced currents (GICs) were recorded in June 1991 to May 1992 in a 400 kV transmission line in southern Finland. Geomagnetic field variations were simultaneously measured at the nearby Nurmijarvi Geophysical Observatory. Two models are presented here to calculate GICs from the time derivative of the magnetic field. A plane wave model with a homogeneous earth is applied by letting the Earth's conductivity be a fitting parameter. Another approach derives experimental impulse responses between GIC and time derivatives. Both methods yield approximately accuracy. When GICs larger than 10 A, 20 A and 30 A are considered the average misfits are ca. 40%, 30%, and 20%, respectively.