Electromagnets and their uses
An electromagnet is a coil of insulated wire with a soft iron core inside it. The coil is made by winding enamelled copper wire in several layers on a frame (of electrical board or fibre), protected outside with insulating paper, with the wire ends brought out to terminals. The core is made of soft steel or an iron alloy with silicon and nickel; it is often built from thin plates insulated from each other. When current passes through the coil winding, the core becomes magnetised and a magnetic field arises and strengthens; when the current stops, the magnetism almost disappears, so an electromagnet can be switched on and off at will. The pulling force on an iron armature (plate) depends on the current and on the number of turns per unit length of the coil: more current or more turns make the magnetic field stronger and the armature is pulled harder (until the core saturates). Electromagnets are used in lifting cranes, electric motors, electric bells, telephones, telegraphs, automation and relays. In experiments only a battery (up to 4.5 V) is used, because an electromagnet and battery heat up if connected for long: connect briefly and then disconnect. Connecting an electromagnet to the 220 V mains is strictly forbidden.
Experiment (supervised): wind enamelled wire on a wooden/plastic bobbin or a nail, connect briefly to a 4.5 V battery and count how many paper clips it lifts; double the turns and test again. Wire and battery may heat: disconnect after every test.