(1) During the experiment, moving the slide of the sliding varistor can change the current in the circuit;
(2) ① Compare 1, 2, 3 (or 4, 5, 6) It can be seen that when the number of turns of the coil is the same, when the current increases from 0.8A to 1.5A, the number of attracted iron nails increases from 5 to 10, which means that when the number of turns of the coil is the same, Timing, the greater the current passing through the electromagnet, the stronger the magnetism of the electromagnet;
② Comparing 1 and 4 (or 2 and 5 or 3 and 6) in the experiment can determine the current passing through the electromagnet When both are 0.8A, the coil with 50 turns attracts 5 iron nails, and the coil with 100 turns attracts 7 iron nails. This shows that when the current passing through the electromagnet is the same, the more turns the coil has, the smaller the electromagnet will be. The stronger the magnetism.
(3) ①The magnetic strength of the electromagnet may be related to the size of the iron core. When exploring the relationship between the magnetic strength of the electromagnet and the size of the iron core, the control current and number of coil turns remain unchanged and the size of the iron core is changed.
② Verification plan: Ensure that the number of coil turns and current in the two circuits are the same, make the sizes of the iron cores inserted twice different, and see the number of iron nails attracted. If the numbers are different, it means that the strength of the magnetism is related to the size of the iron core.
So the answer is: (1) The sliding blade of the sliding rheostat; (2) ① The stronger the magnetism; ② The stronger the magnetism; (3) ① The magnetic strength of the electromagnet may be related to the iron in the coil It is related to the size of the core; ② Ensure that the number of coil turns and current in the two circuits are the same, and let the sizes of the iron cores inserted twice be different to see the number of iron nails attracted. If the number of iron nails attracted is different, it means The strength of magnetism is related to the size of the iron core.