1)The reason for the covering of wire with rubber is that as rubber is insulator it doesn't conduct electricity so if it would not be covered with insulator then anyone can get shock and he would suffer current which can led to his death also.

2)To study magnetic field , an electromagnet can be made using iron nal, wire and battery.For making electromagnet, iron nailed is first coiled with wire and then the wire is connected to a battery. When any magnetic material is brought near the bar magnet, it will got attracted towards the magnet . While any non-magnetic material will not get attracted by it. Thus the effect of magnetic field can be studied that a magnet attracts magnetic material or objects. And bar magnet is a parmanent magnet that will always attract magnetic material, unlike iron nail that required a battery connection to work as a magnet . So magnetic field can be studied by comparing magnetism created using two types of magnet that is parmanent magnet that is bar magnet . And temporary magnet that is electromagnet .