The resistivity 
of a material is the physical property which determines the aptitude of
this material to be opposed to the passage of the electrical current.
The resistivity is the ohmic resistance of a cylinder of unit section
and length.
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with: |
The usual units are the ohm for resistances and the
meter for the lengths. The resistivity unit will be thus the ohm.m. The
ohm.cm employed sometimes is value: 0,01 ohm.m. The conductibility
is the reverse
of the resistivity and is expressed in mho/m. In hydrogeology we
generally employ the micromhos/cm or microsiemens/cm.
(ohm.m) = 10'000 /
(microsiemens/cm) = 1000 / (millisiemens/m)
Resitivity of different formations
Strictly speaking the Ohm law is valid only for metal conductors. For gas and electrolytes it is only an approximation.