4. A solenoid 15 cm long is would in two layers. Each layer contains 100 turns; the first layer is 2 cm in radius, the second 2.05 cm. (a) If the winding carries a current of 3 A, find the magnetic induction at various points along the axis of the solenoid. (b) Make a plot of the axial magnetic induction as a function of distance, from the center to one end of the solenoid.