The electrical resistance of any object
is directly proportional to its length, l, and inversely proportional to its
area of cross section, A That is,
R µ l
A  or R = r l
A
(2 17)
The constant of proportionality, r (Greek, rho), is called resistivity
(specific resistance) Its SI units are ohm metre (W m) and quite often
its submultiple, ohm centimetre (W cm) is also used