Answer: switching the side of the battery to which each side of the wire is connected
Step-by-step explanation:
An electromagnet works on the principle that current generates magnetic field along with the electric field around it. The strength of the magnetic field depends upon the number of turns (N), amount of current (I) and permeability of the material (μ).
B = μ N I
From the given options,
A. Changing type of wire that has low resistance. A low resistance wire would allow greater amount of current which would increase the magnetic field.
B. Making coils in the wire. We know that with increase in the number of turns, the strength of the magnetic field increases.
C. Increasing the number of batteries that are used would increase the voltage which would further increase the current ( I ∝ V ). Hence, the strength of the magnetic field would increase.
D. Switching the side of the battery would only reverse the polarity of the magnetic field but not increase its strength.
Switching the side of the battery to which each side of the wire is connected will NOT increase the magnetic field that is being produced.