This current, in turn, generates a magnetic field that opposes the field produced by the magnet under the table. The aluminum ring becomes, momentarily, a magnet that is repelled by the hidden magnet. The induced field is opposite to the inducing field. If the light bulb is connected to a wire loop that intercepts changing magnetic field lines from an electromagnet, voltage will be induced which can illuminate the bulb. Change is the key, so the electromagnet should be powered with ac.
Induction occurs only for a change in the intercepted magnetic field.
The galvanometer will show a current reading when the current in the first coil is either increasing or decreasing. If the current in the first coil is steady, no current is induced in the secondary and the galvanometer reads zero. The galvanometer needle will swing in opposite directions for increasing and decreasing currents in the primary coil. The iron core increases the magnetic field of the primary coil, as stated in the answer to Exercise 2. The greater field means a greater magnetic field change in the primary, and a greater voltage induced in the secondary.