（214? Taian) As shown in the figure, when the switch is closed and the slider P of the sliding rheostat is moved to the right, the indicator of the spring dynamometer becomes smaller. Then the followi

（214? Taian) As shown in the figure, when the switch is closed and the slider P of the sliding rheostat is moved to the right, the indicator of the spring dynamometer becomes smaller. Then the following analysis is correct. a. When the slider P of the sliding rheostat moves to the right, the resistance in the circuit becomes smaller, and the current in the circuit becomes larger, so it can be determined that the magnetism of the electromagnet becomes stronger;

The lower end of the magnet is N pole, and the indicator of the spring dynamometer becomes smaller.

∵ The magnetic poles with different names attract each other, and the magnetic poles with the same name repel each other.

∴ The upper end of the electromagnet is N pole, so A doesn't meet the question.

B and ∵ Electromagnets have N poles at their upper ends and S poles at their lower ends,

There is no current in the circuit, so the electromagnet has no magnetism.

The electromagnet neither attracts nor repels the bar magnet.

But the bar magnet has gravity, so the spring dynamometer has an indication. Therefore, C does not meet the question.

After D and ∵ remove the iron core, the electromagnet's magnetism becomes weak.

The upper end of the electromagnet is N-pole, and the poles with the same name repel each other. <