(2) According to Ampere's Law, the lower end of solenoid is N pole and the upper end is S pole.
So the answer is: S.
(3) When the slider slides to the right, the resistance of the sliding rheostat decreases, the current in the circuit increases, the magnetism of the electromagnet increases, and the attraction to the iron block increases, so the indicated value of the dynamometer increases.
When wire A is connected to terminal 2 instead of terminal 1, the number of turns of the electromagnetic coil increases, so the magnetism of the electromagnet increases and the indication of the dynamometer increases.
So, the answer is: bigger; Get bigger.
(4) The stronger the magnetism of electromagnet, the greater the attraction to iron, and the greater the indicated value of dynamometer; On the contrary, the pointer of dynamometer is smaller, and we can know the strength of electromagnet magnetism through the pointer of spring dynamometer.
So the answer is: the reading size of the spring dynamometer.