| Material | Resistivity at 20°C | |
|---|---|---|
| Ω·m | µΩ·cm | |
| Elements | ||
| silver | 1.6 × 10-8 | 1.6 |
| copper | 1.7 × 10-8 | 1.7 |
| gold | 2.2 × 10-8 | 2.2 |
| aluminium | 2.7 × 10-8 | 2.7 |
| magnesium | 4.2 × 10-8 | 4.2 |
| tungsten | 5.4 × 10-8 | 5.4 |
| nickel | 6.9 × 10-8 | 6.9 |
| iron | 10.1 × 10-8 | 10.1 |
| chromium | 13.2 × 10-8 | 13.2 |
| manganese | 160 × 10-8 | 160 |
| carbon (graphite) | 3 000 × 10-8 | 3 000 |
| Alloys | ||
| Manganin | 44 × 10-8 | 44 |
| Constantan (Eureka) | 49 × 10-8 | 49 |
| Nichrome | 110 × 10-8 | 110 |
The resistivity values vary depending upon the purity of the sample and the manufacturing process. Copper wire that has been annealed (heated to remove defects in the crystal lattice of the copper) has a slightly lower resistivity.
For metal wires it is best to check the manufacturer's data to find out what the resistivity should be. This is particularly true of resistance wires that can be manufactured with many different compositions to suit different uses.
The resistance of these special alloys only changes by a very small amount as the temperature increases. This is an important feature of the alloy and means that the wires can be used to make accurately known resistors whose resistance will not change much with temperature.
Example data:
Goodfellow: Constantan resistance alloy, Cu55/Ni45
Resistivity: 52 × 10-8 Ω m
Constantan is an alloy used for resistance wires. This example is made from copper (55%) and nickel (45%) with small amounts of other elements.
Constantan is used in resistance boxes and for variable resistors. The name itself is shortened from the words "constant resistance".
Example data:
Goodfellow: Nichrome V, Ni80/Cr20
Resistivity: 108 × 10-8 Ω m
Nichrome is often used for heating elements such as those found in electric fires. Nichrome V is approximately 80% nickel and 20% chromium with small amounts of other elements.
Example data:
Goodfellow: Manganin, Cu86/Mn12/Ni2
Resistivity: 43-48 × 10-8 Ω m
Manganin was found to be very suitable for constructing standard resistors that have accurately known values.
The composition of this sample is 86% copper, 12% manganese and 2% nickel.
ρ = RA/l
There are a number of confusing units for resistivity. This should help you to convert most of the common ones if you are looking up a resistivity value from a web site or technical data book.
The SI unit is the ohm metre.
The microhm centimetre is a popular unit. The length and cross-sectional area are measured in centimetres and square centimetres. The resistance is measured in micro ohms.
1 cm = 1 × 10-2 m
1 µΩ = 1 × 10-6 Ω
1 µΩ·cm = 1 × 10-8 Ω·m
This is an engineering unit that you might come across in American data. The length of the wire is measured in feet. The area is measured in circular mil. A mil is one thousandth of an inch. A circular mil is the area of a circle with a diameter of 1 mil.
1 Ω·CMF = 0.1662 × 10-8 Ω·m
Another engineering unit. This time the area is that of a square with sides of 1 mil.
1 Ω·SMF = 0.2117 × 10-8 Ω·m
©8886 3-Jan-2004