The magnetic field is proportional to the current i(t). However, while the field of the single line decreases only inversely proportional to the distance, the field of the go-and-return line decreases inversely proportional to the square of the distance at large distances. This leads to a dramatically different distance dependence of the inductive coupling for each network model. This behaviour for the magnetic field and for the inductive coupling per unit length is shown in the next figure. The current i(t) is chosen to be of 1 A and the distance a = 1.5 mm.