The resistance for most resistors changes with temperature. The temperature coefficient of electrical resistance is the change in electrical resistance of a resistor per unit change in temperature.

The temperature coefficient of resistance is measured in W/°C. The temperature coefficient of resistors may be either positive or negative.

A positive temperature coefficient denotes a rise in resistance with a rise in temperature; a negative temperature coefficient of resistance denotes a decrease in resistance with a rise in temperature.

Pure metals typically have a positive temperature coefficient of resistance, while some metal alloys such as constantin and manganin have a zero temperature coefficient of resistance.

Carbon and graphite mixed with binders usually exhibit negative temperature coefficients, although certain choices of binders and process variations may yield positive temperature coefficients. The temperature coefficient of resistance is given by

R(T2) =R(T1)[1 +aT1(T2–T1)]

where aT1is the temperature coefficient of electrical resistance at reference temperature T1, R (T2) is the resistance at temperature T2(W), and R(T1) is the resistance at temperature T1(W).

The reference temperature is usuallytaken to be 20°C. Because the variation in resistance between any two temperatures is usually not linear as predicted by Eq, common practice is to apply the equation between temperature increments and then to plot the resistance change versus temperature for a number of incremental temperatures.

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