Temperature Coefficient of Resistance

Temperature Coefficient of Resistance refers to the variation in resistance in response to increase in temperature (per unit kelvin). 

When the temperature of a material is increased, the atoms are agitated due to the increase in heat energy.  This agitation causes an increase in resistance to the flow of electrons.  This causes the resistance of a material to increase.  This is termed as positive coefficient of Resistance.  

The resistance of a material at a given temperature can be determined by the following formula
Where Ro is the resistance of the material at zero degree temperature.  α  is the temperature coefficient of resistance while T is the temperature in kelvin)

Negative Temperature Coefficient of Resistance

Some materials also display a negative temperature coefficient of resistance  i.e. the resistance decreases with increase in temperature.  Carbon, Silicon and Germanium are elements with negative temperature coefficient of resistance. Insulators such as plastics, rubber, etc also exhibit negative temperature coefficient of resistance.