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Encyclopedia of Electrical Engineering
 realnfo.com
Encyclopedia of Electrical Engineering

# Resistance: Metric Units

The design of resistive elements for various areas of application, including thin-film resistors and integrated circuits, uses metric units for the quantities of Eq. (1).
$$R = \rho {l \over A}\,(ohm) \, \text{...............eq.1}$$
$\rho = \text{CM-Ω/ft at T=20 ℃}$
$l = \text{Length(feet)}$
$A= \text{Area (in circular mils (CM))}$
In SI units, the resistivity would be measured in ohm-meters, the area in square meters, and the length in meters. However, the meter is generally too large a unit of measure for most applications, and so the centimeter is usually employed. The resulting dimensions for Eq. (1) are therefore
$\rho$ : ohm-centimeters
$l$ : Length(centimeters)
$A$ : square centimeters
The units for $\rho$ can be derived from $$\rho ={RA \over l }= {Ωcm^2 \over cm} = Ωcm$$