If I is the length of the conductor, A its area of the crosssection and R its total resistance then,
Where P is a constant of proportionality and is called the electrical resistivity of the material of the conductor. The SI unit of resistivity is Ohm meter.
Now, plug the key. Note the current in the ammeter. Now replace the wire by a thicker nichrome wire, of the same length. A thicker wire has a larger cross-sectional area. Again note down the current through the circuit. Instead of taking a nichrome wire, connect a copper wire in the circuit. Again note down the current