The well known mathematician and electrical engineer Charles Proteus Steinmetz published an article with the title "The Polar Diagram of Alternate Currents and Its Application to Inductive Resistances" in the June 20, 1891 issue of the "Electrical Review." In the article he considers an alternating current flowing through an inductive resistance (a coil consisting of 50 turns), and he wrote: "Let σ = 100, the magnetic conductivity of the magnetic circuit produced by the coil: that is: 1 ampere turn produces 100 lines of magnetic force surrounding all the 50 turns of the coil (or 200 lines surrounding only half the coil, etc.)" This is clear except for the last part, "or 200 lines surrounding only half the coil." How did 100 lines of force/"ampere turn" become 200 when considering "half the coil"? And what did he mean by "half the coil"? Can anyone explain this? It is a puzzler!