A conducting straight wire produces a magnetic field around it when current is passed through it due to magnetic effect of current.

To demonstrate the magnetic field produced around such conducting wire by using iron dust, cardboard the following procedures should be followed:

• Keep the conducting straight wire with current passed through it at the middle of the cardboard.
• Lay a thin sheet of paper over the cardboard, directly above the conducting wire. This will act as a surface to hold the iron dust and prevent it from scattering too much.
• Evenly sprinkle a small amount of iron dust over the paper.
• Once the current is flowing, gently tap or shake the paper. 

The iron dust will begin to align itself in a particular manner. The iron dust shows the magnetic field of the conducting straight wire produced by current in the wire.

In this way, magnetic field produced around straight current carrying conducting wire be demonstrated by using iron dust and cardboard.