(2004?Tianjin) is a schematic diagram of an electromagnet with a tap in the center, Xiaoming in order to study the shape of the same electromagnet magnetic strength and energized electricity.

① (1) and (2) control is a certain number of turns of the coil, the study of the electromagnet magnetic strength and the relationship between the size of the current. When the current increases, the number of pins attracted by the electromagnet increases, indicating increased magnetism. Conclusion: a certain number of turns of the coil, the greater the current, the stronger the magnetism of the electromagnet;

② (2) and (3) control is a certain current, the study of the strength of the magnetism of the electromagnet with the number of turns of the coil of the relationship. When the number of turns of the coil increases, the number of pins attracted by the electromagnet increases, indicating increased magnetism. Conclusion: when the current is certain, the more turns of the coil, the stronger the magnetism of the electromagnet.

(4) because the electromagnet magnetic strength and current size and the number of turns are related to the coil, so in the study of the relationship between the strength of the magnetism and the number of turns of the coil, it is necessary to ensure that the current must be a certain condition to study.

The answer is (3) ① coil turns, the greater the current, the stronger the magnetism of the electromagnet; ② current is certain, the more turns of the coil, the stronger the magnetism of the electromagnet.

(4) to ensure that under certain conditions of current, the study of the relationship between the strength of the magnetism of the electromagnet and the number of turns of the coil.