(2012?Foshan) As shown in the figure, A is a magnet suspended from the lower end of a spring, B is the core of a solenoid, and S is a single-pole, double-throw switch that remains in contact with 2. T

(2012?Foshan) As shown in the figure, A is a magnet suspended from the lower end of a spring, B is the core of a solenoid, and S is a single-pole, double-throw switch that remains in contact with 2. To make According to Ampere's rule, the top end of the solenoid is the N pole, the bottom end is the S pole, so the energized solenoid and the magnets on it are mutually exclusive,

A, the solenoid's iron core B drawn out, its magnetic properties become weaker, so the length of the spring becomes bigger, so the A is wrong;

B, the slider P will be moved to the right of the sliding rheostat, the resistance becomes bigger, the current becomes smaller, the magnetic properties become weaker, so the length of the spring becomes bigger, so the B is wrong. The length of the spring becomes larger, so B is wrong;

C, will be changed from 2 to 1, the number of turns of the coil becomes more, the magnetism becomes stronger, so the length of the spring becomes shorter, so C is correct;

D, to reduce the number of a dry-cell battery, so the current becomes smaller, the magnetism becomes weaker, and the length of the spring becomes larger, so D is wrong;

Therefore, the selection of the C.

The length of the spring becomes larger, so the length of the spring becomes smaller, so the length of the spring becomes shorter.