You put a DC electro-magnet to hang on a cable arrange power depending on desired lifting force (usually 10 to 100 amps DC the crane needs to be sized for lifting weight of iron by magnet. Crane needs to be able to rotate ( move the iron from one place to another ) cooling will be a concern, so a tempera monitor will be useful to indicate when to stop the magnet and let it cool down before further lifting.. I can help you design the magnet , the steel crane mechanism, the power supply and the controls including cooling as needed.
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you can make an electromagnet by coiling a large amount of wire around a bit of iron bar (or steel), and connecting it up to a power supply that can deal with a fair amount of current. A car battery is ideal for larger loads, over 1Kg, and a set of C or D cells are suitable for smaller magnets.If you need to design the actual crane, a drag-line style setup would be easiest on a toy/model, though if you need to actually lift a reasonable load, an engine hoist or small gantry crane would be a good place to start.