How does a scrap heap magnet work? (in detail)

is nothing but a heavy electromagnet. On a soft iron core (because, there should not be any retentivity) thick insulated copper wire is wound. As the magnetic power of an electromagnet is depending upon the current, number of turns in the coil, the copper wire should be able to withstand heavy current and more number of turns will create very strong magnetic field. When the current passes it will become a very strong magnet and will attract all the magnetic materials in the scrap. When there is no current the core gets dememagnetised and all the magnetic materials will be falling down from it . This arrangement is fitted at the tip of the cable of a crane and the operating switch will be near the operators cabin. The crane will be brought near the scrap, and the operator switch the magnet on and all the scrap will be attracted by the magnet. Then the crane will be turned to another place the operator switches the magnet off. All the scraps will be falling down. 
Instead of a soft iron core ( less retentivity) anyother material with high retentivity is used even after switching it off the scrap will not bec falling down.