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Resistivity and temperature coefficient of resistance
- table or chart of the resistivity and temperature coefficient of resistance of a variety of materials many of which are used in electronics equipment.
Two important electrical features of any substance are its resistivity and its temperature coefficient of resistance. These two properties are particularly important and will often determine whether a substance can be used in the manufacture of a wide variety of electrical and electronics components from wire to components such as resistors, potentiometers and many more.
Resistivity
The electrical resistivity of a material is also known as its specific electrical resistance. It is a measure of how strongly a material opposes the flow of electric current. The resistivity is the electrical resistance per unit length and per unit of cross-sectional area. This is for a particular material at a specified temperature. It is also possible to define the resistivity of a substance as the resistance of a cube of that substance having edges of unit length, with the understanding that the current flows normal to opposite faces and is distributed uniformly over them. The SI unit for electrical resistivity is the ohm metre, although it is also sometimes specified in ohm centimetres.
This means that a low resistivity indicates a material that readily allows the movement of electrons. Conversely a high resistivity material will have a high electrical resistance and will impede the flow of electrons.
Elements such as copper and aluminium are known for their low levels of resitivity. Silver and in particular, gold have a very low resistivity, but for obvious cost reasons their use is restricted.
Temperature coefficient of resistance
The temperature coefficient of resistance, often designated alpha, is defined as the amount of change of the resistance of a material for a given change in temperature. A positive value of alpha indicates that the resistance increases with temperature; a negative value of alpha indicates the resistance decreases; and a value of zero a indicates that the resistance is constant. For most metals it is found that the resistance increases with temperature, whereas the opposite is true for semiconductor where the resistance falls with increasing temperature.
| Substance |
Resistance at 0C
Ohm metres |
Temperature Coefficient
K-1 |
| Aluminium |
0.25 x 10-6 |
38 x 10-4
(18C - 100C) |
| Antimony |
3.9 x 10-6 |
40 x 10-4 |
| Bismuth |
10.6 x 10-6 |
42 x 10-4 |
| Brass |
~0.6 - 0.9 x 10-6 |
10 x 10-4 |
| Cadmium |
0.60 x 10-6 |
40 x 10-4 |
| Cobalt |
0.56 x 10-6 |
33 x 10-4 |
| Copper |
0.16 x 10-6 |
43 x 10-4 |
| Ebonite |
2 x 1014 |
-- |
| German silver |
1.6 - 4.0 x 10-6 |
Approx 4.5 x 10-4 |
| Glass |
1013 |
-- |
| Gold |
0.20 x 10-6 |
40 x 10-4 |
| Graphite |
300 x 10-6 |
-5.6 x 10-4 |
| Iron |
0.89 x 10-6 |
62 x 10 -4 |
| Lead |
1.9 x 10-6 |
43 x 10-4 |
| Manganin |
4.2 x 10-6 |
~0.1 x 10-4 |
| Mica |
9 x 1014 |
-- |
| Nickel |
0.61 x 10-6 |
27 x 10-4 |
| Palladium |
1.0 x 10-6 |
37 x 10-4 |
| Phosphor-bronze |
0.5 - 1.0 x 10-6 |
-- |
| Platinum |
0.98 x 10-6 |
38 x 10-4 |
| Quartz |
1 x 1013 |
-- |
| Silver |
0.15 x 10-6 |
40 x 10-4 |
| Tantalum |
1.3 x 10-6 |
33 x 10-4 |
| Tin |
1.1 x 10-6 |
45 x 10-4 |
| Tungsten |
0.49 x 10-6 |
51 x 10-4 |
| Zinc |
0.55 x 10-6 |
36 x 10-4 |
Applications
Many of the materials found in the list above are widely used in electronics. Aluminium and particularly copper are used for their low levels of resistance. Most wire used these days for interconnections is made from copper as it offers a low level of resitivity at an acceptable cost. Gold while much better is more costly and is used in much smaller quantities. Often gold plating is found on high quality low current connectors where it ensures the lowest contact resistance. Silver is not so widely used because it tarnishes and this can result in higher contact resistances. The oxide can also under some circumstances act as a rectifier which may cause some annoying problems in RF circuits.
Tantalum is used in capacitors, and nickel and palladium are used in the end connections for many surface mount components such as capacitors. Quartz finds its main use as a piezo electric resonant element. Quartz crystals are sued as frequency determining elements in many oscillators where its high value of Q enables very frequency stable circuits to be made. They are similarly used in high performance filters.
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