Specific production process (1) Material preparation: beverage bottle, three-gear switch, two dry batteries, enameled wire, several thin wires, strip magnetic core (made of silicon steel sheet), insulating tape, transparent tape and welding tools. (2) Making a plastic shell similar to a "bar magnet": Cut a section of the discarded beverage bottle with a pair of scissors, measure the size of the bar magnet with a ruler, and make two mutually nested shells according to half the size of the bar magnet, one for making a coil and the other for installing a switch and a power supply. (3) coil making: tightly wound with enameled wire on the shell. In order to ensure strong magnetism, arrange them as closely as possible and increase the number of turns. Stick the iron core in the middle of the plastic casing with adhesive tape, and then fix all windings with transparent tape. In order to make it more magnetic, you can also wind several layers on the first layer of enameled wire, and also wind the fixed winding with adhesive tape. (4) Install the switch: cut a small opening with a knife at a suitable position on another shell. The small opening should be slightly smaller than the size of the switch, so as to just clamp the switch and leave space on the right side of the switch to place the power supply. (5) Manufacture of power supply: In order to realize greater magnetic and current commutation, two 12 V batteries are used in the teaching aid. The anode of one battery is connected with the cathode of another battery and welded firmly, and the other two poles are reserved for connecting the three-speed switch terminals. (6) Connecting circuit: according to the schematic diagram of the circuit, connect the wires of all parts, weld the joint firmly with welding tools, and wrap the wire ends and exposed parts with insulating tape to prevent short circuit of power supply. Socket the plastic shell of the coil with the plastic shell of the switch and power supply, and stick it firmly with transparent tape or glue. (7) Making the "Magic Revealing" outer box: making the outer box with the shape of bar magnets with opaque paper shells, and putting the self-made teaching AIDS into the box, leaving only a small opening for operating the switch. When operating, use your thumb to block the switch, so that you can't see the equipment, paving the way for "revealing the secret" in the experiment. Source: Discussion on Physics Teaching