1.4.2 Steepness of lightning current rise

Electric currents are surrounded by a magnetic field whose field strength is proportional to the current strength. A change in the current over time Δi/Δt therefore also causes a change in the magnetic field over time. Magnetic fields that change over time and/or in terms of location induce voltages in all open or closed conductor loops. The faster the magnetic field affecting the conductor loop changes, the greater the induction voltage. 

The steepness of the lightning current rise Δi/Δt, which is effective during the interval Δt, thus determines the level of the electromagnetically induced voltages. 

The approximately rectangular voltage U induced in a conductor loop during the interval Δt is (Equation 1.4.2.1):