To boost the power of his electromagnet and attract at least four paper clips, Seamus should consider the following:
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Increase the voltage: By adding more batteries (in series) to increase the voltage supplied to the wire, he can increase the current flowing through the wire, which will enhance the magnetic field produced by the coils.
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Decrease the space between the wire coils: By tightly winding the wire around the nail and decreasing the space between the coils, he can increase the strength of the magnetic field generated because the magnetic field lines will be more concentrated.
Thus, the best choice for Seamus would be:
Seamus can add batteries to increase the voltage, and he can decrease the space between the wire coils.