What is electrical resistivity? In a series electrical circuit comprising of a resistor having a metallic wire, the ammeter reads 5 A. The reading of the ammeter decreases to half when the length of the wire is doubled. Why?
Answer:
The resistivity of a material is defined as the resistance of a conductor made of that material of unit length and unit cross-sectional area.
Using, 
R is doubled while V remains unchanged. Hence, current becomes half.