(1) In the experiment, the magnetic strength of the electromagnet is judged by how many pins it attracts.
So the answer is: How many pins does the electromagnet attract?
(2) The magnetic strength of the electromagnet is related to the size of the current in the coil, the number of turns of the coil and the condition of the iron core.
So the answer is: ①The number of turns of the coil; ②The size of the current passing through the coil; ③The size of the iron nail.
(3) Since the magnetic strength of the electromagnet is related to the above two factors, we must use the controlled variable method when exploring; for example, when exploring the relationship between current, voltage and resistance, we must We have reached this method;
So the answer is: control variable method; explore the relationship between current and voltage.
(4) Electronic banknote detectors mainly use fluorescence for detection, so A does not meet the meaning of the question.
The model motor is made based on the principle that the energized coil is forced to rotate in the magnetic field, so B does not meet the meaning of the question.
The main component of the electromagnetic relay is the electromagnet, which works based on the principle that the electromagnet is magnetic when energized and non-magnetic when powered off. So C fits the meaning of the question.
The hand-operated generator is made based on the principle of electromagnetic induction, so D does not meet the meaning of the question.
So choose C.