Demonstration: Field Lines of a Wire

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  • Car battery (12 V)
  • Straight wire
  • Piece of white cardboard
  • Connecting wires
  • Knife switch
  • Iron fillings

The two ends of a long wire are connected to a 12 V car battery through a high capacity knife switch. The magnetic field lines near the wire are demonstrated with the aid of iron fillings sprinkled over a cardboard surface. When the switch is closed for a short time the iron fillings are caused to line up in circles around the wire. (It is important to immediately reopen the switch to prevent overheating.)