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CIRCULAR-MIL-FOOT | Up
Neets Module 04-Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading | Next
WIRE SIZES |
1-5
offered by a cube-shaped conductor 1centimeter in length and 1 square centimeter in cross-sectional area.
The unit of volume to be used is given in tables of specific resistances.
SPECIFIC RESISTANCE OR RESISTIVITY
Specific resistance, or resistivity, is the resistance in ohms offered by a unit volume (the circular-mil-
foot or the centimeter cube) of a substance to the flow of electric current. Resistivity is the reciprocal of
conductivity. A substance that has a high resistivity will have a low conductivity, and vice versa. Thus,
the specific resistance of a substance is the resistance of a unit volume of that substance.
Many tables of specific resistance are based on the resistance in ohms of a volume of a substance 1
foot in length and 1 circular mil in cross-sectional area. The temperature at which the resistance
measurement is made is also specified. If you know the kind of metal a conductor is made of, you can
obtain the specific resistance of the metal from a table. The specific resistances of some common
substances are given in table 1-1.
Table 1-1.—Specific Resistances of Common Substances
Specific resistance at 20ºC.
Substance
Centimeter cube
(microhoms)
Circular-mil-foot
(ohms)
Silver
1.629
9.8
Copper (drawn)
1.724
10.37
Gold
2.44
14.7
Aluminum
2.828
17.02
Carbon (amorphous)
3.8 to 4.1
……………………
Tungsten
5.51
33.2
Brass
7.0
42.1
Steel (soft)
15.9
95.8
Nichrome
109.0
660.0
The resistance of a conductor of a uniform cross section varies directly as the product of the length
and the specific resistance of the conductor, and inversely as the cross-sectional area of the conductor.
Therefore, you can calculate the resistance of a conductor if you know the length, cross-sectional area,
and specific resistance of the substance. Expressed as an equation, the "R" (resistance in ohms) of a
conductor is
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