Physically, the resistivity
for a substance is its resistance when it is one metre long and its area
of cross section is one m2 It can be seen that:
1 W m = 100 W cm or 1 W cm = 0 01 W m
The inverse of resistance, R, is called conductance, G, and we have
the relation:
G = 1
R = ρ
A=κ
A
l
l
(2 18)
The SI unit of conductance is siemens, represented by the symbol
‘S’ and is equal to ohm–1 (also known as mho) or W–1