CHANGE OF RESISTANCE WITH TEMPERATURE

Resistance of all materials is affected by changes in temperature. Non-conductors or insulators usually decrease in resistance with an increase in temperature. 

For example - glass is an insulator when cold, and become a fairly good conductor when red hot.

Mostly metallic conductors increase in resistance with an increase in temperature. This is for all metals. Alloys differ in this characteristic, some showing practically no change in resistance with the change of temperature. Manganin (an alloy of copper manganese and nickle) changes its resistance by 0.001% per degree centigrade. Carbon decreases in resistance with increase in temperature.

If the resistance of pure meal is plotted against temperature (see figure), it is found that the graph is practically a straight line for a range say 0° C.

If this curve is extended to the left, it cuts the temperature axis at a certain temperature say - T₀ degree.

This point is found to be -234.5° C for annealed copper of 100% conductivity.

This does not mean copper has zero resistance at this temperatures around -234.5° C, through very low.

In practice, the resistance-temperature graph (or characteristics) deviates from the straight line by a marked degree at a very low as well as high temperatures.

Within the usual working range of temperature, however, a straight line relation may be assumed with little error. (The value of T₀ for aluminium is -228.2° C.)

CONDUCTANCE AND UNIT OF CONDUCTANCE - MHO

Conductance: Conductance is a measure of the ease with which the current will flow through a substance.

Conductance is usually denoted by G.

So, this is the reciprocal of resistance, 

i.e.                                

Appropriately, the unit of conductance is termed mho() i.e. the unit of resistance (ohm) spelled backwards.

EFFECT OF TEMPERATURE ON RESISTANCE OF VARIOUS TYPES OF MATERIALS

The effect of rise of temperature on electrical resistance is not always an increase in resistance but sometimes there is a decrease in resistance also.

The effect of temperature on different types of materials can be classified as follows:

(i) Pure Metals: With a rise in temperature, the resistance of pure metals increases and ∝, the temperature coefficient of resistance is said to be positive.

(ii) Insulators, Electrolytes, etc: In the case of insulators, semi-conductors and electrolytes the electrical resistance decreases with a rise in temperature and ∝ in such cases is said to be negative.

(iii) Alloys: The resistivity of an alloy is usually considerably greater than that of its constitute metals and its ∝ is ordinarily very small and irregular. For many purposes the temperature coefficient of resistance of alloys such as Eureka (Cu  60% + Ni  40%) and Manganincalloy of Copper + Magnganese + Nickle) and constantine is taken as negligible.