(a)[3]
As shown in Fig. 6.1, two wires are placed side by side and carry equal currents $I$ in opposite directions. The magnetic flux density produced by the current in the wires is negligible.
(b)[4]
In Fig. 6.2, an air-cored solenoid is connected in series with a battery. When an iron core is inserted into the solenoid, an e.m.f. opposing the battery e.m.f. is induced in the solenoid.