Current is Simply the rate of flow of charge per unit time , nut if we are talking about electromagnetic current here we come across the strange behavior of current . which states when ever charge flows in closed loops it creates magnetic field .
You think correct about the ferromagnetic and its effects.
It plot the graph we must find a relation between them which is given by BiotSarvat law which states that magnetic field B due to current carrying wire at certain distance r from it about its length dl has relation
so we can combine it to form
which gives linear graph hence your point is proved