## Resistivity.

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

MathML formula | Formula Image | Straight text formula |
---|---|---|

$R=\frac{\rho L}{A}$ | 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 10^{14} |

Bakelite | 2 x 10^{5} - 2 x 10^{14} |

Glass | 10^{10} - 10^{14} |

Hard rubber | 10^{13} - 10^{16} |

Mica | 10^{11} - 10^{15} |

Quartz | 75 x 10^{16} |

Sulfur | 10^{15} |

Wood | 10^{8} - 10^{11} |

## 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.

1.0 | 1.2 | 1.5 |

1.8 | 2.2 | 2.7 |

3.3 | 3.9 | 4.7 |

5.6 | 6.8 | 8.2 |

1.0 | 1.1 | 1.2 | 1.3 | 1.5 | 1.6 |

1.8 | 2.0 | 2.2 | 2.4 | 2.7 | 3.0 |

3.3 | 3.6 | 3.9 | 4.3 | 4.7 | 5.1 |

5.6 | 6.2 | 6.8 | 7.6 | 8.2 | 9.1 |

1.00 | 1.02 | 1.05 | 1.07 | 1.10 | 1.13 | 1.15 | 1.18 |

1.21 | 1.24 | 1.27 | 1.30 | 1.33 | 1.37 | 1.40 | 1.43 |

1.47 | 1.50 | 1.54 | 1.58 | 1.62 | 1.65 | 1.69 | 1.74 |

1.78 | 1.82 | 1.87 | 1.91 | 1.96 | 2.00 | 2.05 | 2.10 |

2.15 | 2.21 | 2.26 | 2.32 | 2.37 | 2.43 | 2.49 | 2.55 |

2.61 | 2.67 | 2.74 | 2.80 | 2.87 | 2.94 | 3.01 | 3.09 |

3.16 | 3.24 | 3.32 | 3.40 | 3.48 | 3.57 | 3.65 | 3.74 |

3.83 | 3.92 | 4.02 | 4.12 | 4.22 | 4.32 | 4.42 | 4.53 |

4.64 | 4.75 | 4.87 | 4.99 | 5.11 | 5.23 | 5.36 | 5.49 |

5.62 | 5.76 | 5.90 | 6.04 | 6.19 | 6.34 | 6.49 | 6.65 |

6.81 | 6.98 | 7.15 | 7.32 | 7.50 | 7.68 | 7.87 | 8.06 |

8.25 | 8.45 | 8.66 | 8.87 | 9.09 | 9.31 | 9.53 | 9.76 |

## 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 | - |