I also wanted to know the effect of increase in temperature on the resistance of alloys ?

Question

How to define Temperature Coefficient of Resistance of a conducting or insulating material ?

Answer 1

The average pressure coefficients to 12000 kg/cm2 of electrical resistance for three series of alloys: lithium-tin, bismuth-tin, and calcium-lead, and for one calcium-magnesium alloy of 10 A% magnesium are given in tabular form. The variation with concentration of the pressure coefficient, the temperature coefficient, and the relative change of the temperature coefficient with pressure for these three series is comparable with the variation of the structure of the alloys as given by the equilibrium diagram. The conductivity and the pressure coefficient of a dilute solid solution of bismuth in tin are the same as those of pure tin under pressure. As hydrostatic pressure and impurities have the same effect on the pressure coefficient, electric conduction probably depends on geometrical properties of the conductor.

Temperature coefficient of resistance

You might have noticed on the table for specific resistances that all figures were specified at a temperature of 20o Celsius. If you suspected that this meant specific resistance of a material may change with temperature, you were right!

Resistance values for conductors at any temperature other than the standard temperature (usually specified at 20 Celsius) on the specific resistance table must be determined through yet another formula:

The "alpha" (α) constant is known as the temperature coefficient of resistance, and symbolizes the resistance change factor per degree of temperature change. Just as all materials have a certain specific resistance (at 20o C), they also change resistance according to temperature by certain amounts. For pure metals, this coefficient is a positive number, meaning that resistance increases with increasing temperature. For the elements carbon, silicon, and germanium, this coefficient is a negative number, meaning that resistance decreases with increasing temperature. For some metal alloys, the temperature coefficient of resistance is very close to zero, meaning that the resistance hardly changes at all with variations in temperature (a good property if you want to build a precision resistor out of metal wire!).

Most conductive materials change specific resistance with changes in temperature. This is why figures of specific resistance are always specified at a standard temperature (usually 20o or 25o Celsius).

The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance. This factor is represented by the Greek lower-case letter "alpha" (α).

A positive coefficient for a material means that its resistance increases with an increase in temperature. Pure metals typically have positive temperature coefficients of resistance. Coefficients approaching zero can be obtained by alloying certain metals.

A negative coefficient for a material means that its resistance decreases with an increase in temperature. Semiconductor materials (carbon, silicon, germanium) typically have negative temperature coefficients of resistance.