Specific Resistance and Conductance
Description and calculation of specific resistance and conductance
Specific Resistance
Specific resistance refers to the resistance of a wire made of a specific material \(1\) meter long and \(1 mm^2\) in crosssection.
Wires of equal length and same crosssection made of different materials have different resistances.
Wires of equal length and same cross section of the same material have the same resistance at the same temperature
The specific resistance has the symbol \(ρ\) (rho)
Electrical Conductance
The reciprocal of the resistivity is the electrical conductivity or the conductance.
The specific conductance indicates which length of a wire, with a cross section of \(1\) \(mm^2\), gives the resistance of \(1Ω\).
The symbol of the conductance is \(σ\) (sigma)
\(\displaystyle σ=\frac{1}{ρ}\)
The resistivity is the reciprocal of the specific conductance.
\(\displaystyle ρ=\frac{1}{σ}\)
Calculate Resistance
The resistance of a wire is proportional to its length. A triple wire length also gives a triple resistance.
\(\displaystyle R ∝ l\)
The resistance of a wire is inversely proportional to its cross section. A double cross section gives half the resistance.
\(\displaystyle R ∝\frac{1}{A}\)
Using the resistivity you get the formula:
\(\displaystyle R =\frac{ρ·l}{A}\)
Using the specific conductance, you get the formula:
\(\displaystyle R =\frac{l}{σ· A}\)
Specific resistance and the specific conductance of various conductive materials
Material 
Resistivity ρ 
Conductance σ 
Silver  0.016  62.5 
Copper  0.018  56 
Gold  0.023  44 
Aluminium  0.029  35 
Magnesium  0.045  22 
Tungsten  0.055  18 
Zinc  0.063  16 
Nickel  0.08 ... 0.011  13 ... 9 
Iron  0.10 ... 0.15  10 ... 7 
Tin  0.11  9 
Platinum  0.11 ... 0.14  9 .. 7 
Lead  0.21  4.8 
Mercury  0.96  1.04 
Bismuth  1.2  0.83 
Carbon  100  0.01 
Resistivity of various insulating materials
Material  Resistivity ρ 
Amber  10^{18} 
Glas  10^{14} 
Mica  10^{14} ... 10^{17} 
Rubber  10^{15} 
Hard rubber  10^{12} ... 10^{18} 
Ceramics  10^{12} 
Resin  10^{8} ... 10^{14} 
Marble  10^{9} ... 10^{11} 
Paraffin  10^{16} 
Polystyrene  10^{18} 
Pressboard  10^{10} 
Porcelain  10^{14} 
Shellac  10^{16} 
