Resistivity.

The resistance of a homogeneous conductor of constant cross section is given by:

MathML formulaFormula ImageStraight text formula
R = ρL A resistivity-formula.PNG R = ρL / A

Where

R = resistance, Ohms

L = length of conductor, meters

A = conductor cross-sectional area, meters squared

ρ = resistivity, Ohm-meters

Some values for resistivity are given below:

Material ρ (ohm-meters)
Aluminium 2.63 x 10-8
Brass 6 - 8
Carbon 3500
Constantan (Cu 60, Ni 40) 49
Copper (annealed) 1.72
Iron 10
Lead 22
Manganin (Cu 84, Mn 12, Ni 4) 44
Mercury 94
Nichrome 100
Silver 1.47
Tungsten 5.51
Amber 5 x 1014
Bakelite 2 x 105 - 2 x 1014
Glass 1010 - 1014
Hard rubber 1013 - 1016
Mica 1011 - 1015
Quartz 75 x 1016
Sulfur 1015
Wood 108 - 1011

Preferred Values of Resistors.

Resistors are manufactured to a certain tolerance. The actual value of a resistor can, therefore, be within a certain range. For this reason there are 'preferred values' of resistors. The following tables show three series, the E12, E24 and E96 series. These are so named because they define respectively 12, 24 and 96 values in each decade of resistance.

E12 Preferred Values: (for 10% tolerance resistors)
1.01.21.5
1.82.22.7
3.33.94.7
5.66.88.2

 

E24 Preferred Values:
1.01.11.21.31.51.6
1.82.02.22.42.73.0
3.33.63.94.34.75.1
5.66.26.87.68.29.1

 

E96 Preferred Values:
1.001.021.051.071.101.131.151.18
1.211.241.271.301.331.371.401.43
1.471.501.541.581.621.651.691.74
1.781.821.871.911.962.002.052.10
2.152.212.262.322.372.432.492.55
2.612.672.742.802.872.943.013.09
3.163.243.323.403.483.573.653.74
3.833.924.024.124.224.324.424.53
4.644.754.874.995.115.235.365.49
5.625.765.906.046.196.346.496.65
6.816.987.157.327.507.687.878.06
8.258.458.668.879.099.319.539.76

Resistor Color Codes.

Resistor Color Codes.

Colour Value Multiplier
black 0 x 1
brown 1 x 10
red 2 x 100
orange 3 x 1,000
yellow 4 x 10,000
green 5 x 100,000
blue 6 x 1,000,000
violet 7 x 10,000,000
grey 8 x 100,000,000
white 9 -