 The property of a substance due to which it opposes (or restricts) the flow of electricity (i.e., electrons) through it.  The flow of charge through any material encounters an opposing force similar in many respects to mechanical friction.  The unit of measurement of resistance is the ohm, for which the symbol is Ω.  A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.

 The resistance of any material with a uniform cross- sectional area is determined by the following four factors: 1. Material 2. Length 3. Cross-sectional area 4. Temperature  The resistance R offered by a conductor depends on the following factors : (i) It varies directly as its length, l. (ii) It varies inversely as the cross-section A of the conductor. (iii) It depends on the nature of the material. (iv) It also depends on the temperature of the conductor.

OR Where is a constant depending on the nature of the material of the conductor and is known as its specific resistance or resistivity  unit of resistivity is ohm-metre ( -m). OR Where is a constant depending on the nature of the material of the conductor and is known as its specific resistance or resistivity  unit of resistivity is ohm-metre ( -m).

 The effect of rise in temperature is : to increase the resistance of pure metals,  metals have a positive temperature co-efficient of resistance.  increase is large and fairly regular for normal ranges of temperature To increase the resistance of alloys,  the increase is relatively small and irregular.  the increase in resistance is (or can be made) negligible over a considerable range of temperature. to decrease the resistance of electrolytes, insulators (such as paper, rubber, glass, mica etc.) and partial conductors such as carbon. Hence, insulators are said to possess a negative temperature-coefficient of resistance

 For good conductors, an increase in temperature will result in an increase in the resistance level. consequently, conductors have a positive temperature coefficient.  For semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficients.