Changes Of resistance with temparature:
The resistance of all materials changes with changes in temperature. The resistance of all pure metal increases with temperature. The resistance of electrolytes, insulators, carbon and semi-conductors decreases with increasing temperatures.
If it is assumed that the resistance change is in proportion to the temperature change, then the ratio provides an indication of the material behaviour. It is necessary however, to relate the change of resistance to its initial value. A large value resistor will change its value more than a small value resistor for the same temperature change.
Suppose the resistance of a material at 0ºC (to) is Ro
and at some other temperature (t) the resistance is Rt
the change of resistance is Rt - Ro.
But the change of resistance is per unit value of the original resistance is given by;
this resistance change has been brought about by a temperature change t equal to t -to (to being 0º).
Hence the change in resistance, caused by a 1ºC change in temperature is;
R/T = = because t = 1 and to = 0
This ratio is called the temperature co-efficient of resistance.