How does the heating of a resistor have an effect on its resistance?

The resistance of a resistor is given by the
formula:

Rt = Rref (1 + A*(T - Tref)), where Rref is the
resistance at the reference temperature, usually taken as 20 degree Celsius. And A is
the temperature coefficient of resistance of the material the resistor is made
of.

The resistance of a resistor can either increase or
decrease with the temperature based on the value of A. It is usually positive for
metals.

To maintain the resistance as close as the
reference resistance it is essential to keep the temperature as close to the reference
temperature. The heat generated when current passes through the resistor should be
allowed to escape. It may also be beneficial to turn off the current at regular
intervals and use ways of cooling the circuit like fans.