There are three factors that affect the resistance of a specimen of
material:
(a) the temperature
(b) the dimensions of the specimen - the smaller the cross
sectional area and the longer the specimen the larger the resistance
(c) the material from
which the specimen is made
The property of the material that affects its resistance is
called the resistivity of the material and is given the symbol ρ.
Material | Resistivity (Ωm) | Material | Resistivity (Ωm) | |
Copper | 1.69x10^{-8} | Non-metals | 10^{4} | |
Nichrome | 130x10^{-8} | Insulators | 10^{13} - 10^{16} | |
Aluminium | 3.21x10^{-8} | Germanium | 0.65 | |
Eureka | 49x10^{-8} | Silicon | 2.3x10^{-5} | |
Lead | 20.8x10^{-8} | Carbon | 33-185x10^{-6} | |
Manganin | 44x10^{-8} | Silver | 1.6x10^{-8} |
The resistivities of solutions cannot be quoted generally because they depend on the concentrations
and are therefore variable quantities. However, as an example, the resistivity of pure water is about
2.5x10^{5} Ωm and that of a saturated solution of sodium
chloride about 0.04 Ωm at 20^{o}C.
The reciprocal of
resistivity is known as the conductivity of the material (s)