I have just done an experiment on finding the reactance of (a)an air-core inductor and (b)an inductor with a soft-iron core. I have plotted a graph of Vrms against Irms for each experiment. For the graph in (a), the straight line can pass through origin; yet, for the graph in (b), the straight line cannot pass through the origin, but have a negative y-intercept. However, I cannot find an answer to explain this. Could anyone please kindly tell me why the graph in (b) cannot pass through the origin??