Resistance of constantan wire

The current in a wire is due to the motion of free electrons within the wire. These electrons are not bound to any particular atom but are free to 'wander' through the body of the material.

The more free electrons per unit volume the greater the current for a given voltage difference across the specimen. So for specimens of different material of equal shape and size the one with the fewer free electrons per unit volume will have the highest resistance. The ease with which these free electrons can move through the metal also affects its resistance and this is determined by its crystal structure.

Constantan is an alloy with a low free electron concentration and so the resistance of a given size and shape of specimen will be relatively large.

Note: Constantan is an alloy comprising copper (60%) and nickel (40%) and has a resistance that changes very little with temperature.