The temperature co-efficient of resistance:

The temperature co-efficient of resistance is defined as;

          The Fractional change in resistance from 0ºC, per degree temperature change.

and may be represented graphically as shown below.

The graph is reasonably linear for many materials over a moderate temperature range (0º - 200ºC).

The units are ºC because the ohms cancel out in the calculation.

Materials whose resistance increases with increasing temperature have a positive temperature co-efficient of resistance.

Materials whose resistance decreases with increasing temperature have a negative temperature co-efficient of resistance.

Some materials have very small temperature co-efficients of resistance and are used where it is important that the resistance does not change with temperature.  Examples are Manganin and Eureka.