and 
:
of: a material (in a given state)
is: the mean fractional change of resistivity per unit temperature rise between and :
is defined more generally: over any suitable temperature range, by applying the linear relation
where 
is the resistivity at temperature 
and 
is the resistivity at temperature 
[P11.4]
generally is: positive for a metal, due to the increase in lattice vibrations with increasing temperature. [P11.4]
often is: negative for a semiconductor, due to increase in the number of mobile charged particles with increasing temperature. [P11.4]
Copyright 1997, The Open University