②(2) and (3) control constant current, and study the relationship between the magnetic field strength of electromagnet and the number of turns of coil. When the number of turns of the coil increases, the number of needles attracted by the electromagnet increases, indicating that the magnetism is enhanced. Conclusion: When the current is constant, the more turns of the coil, the stronger the magnetism of the electromagnet.
(4) Because the magnetic field strength of electromagnet is related to the current and the number of turns of the coil, it is necessary to study the relationship between the magnetic field strength and the number of turns of the coil under a certain current.
So the answer is (3)① When the number of turns of the coil is fixed, the greater the current, the stronger the magnetism of the electromagnet; ② When the current is constant, the more turns of the coil, the stronger the magnetism of the electromagnet.
(4) Under the condition of constant current, the relationship between electromagnet magnetic field strength and coil turns is studied.