Resistance of constantan wire
As you may know 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.