The problem with using resistance as a measurement is that it depends not only on the material out of which the wire is made, but also the geometry of the wire. If we were to increase the length of wire, for example, the measured resistance would increase. Also, if we were to decrease the diameter of the wire, the measured resistance would increase. We want to define a property that describes a material's ability to transmit electrical current that is independent of the geometrical factors.
The quantity that is used is called resistivity and is usually indicated by the Greek symbol rho*,**.
Resistivity is a fundamental parameter of the material making up the wire that describes how easily the wire can transmit an electrical current. High values of resistivity imply that the material making up the wire is very resistant to the flow of electricity. Low values of resistivity imply that the material making up the wire transmits electrical current very easily.
*Unfortunately, the symbol rho is used throughout the geophysical literature to represent both density and resistivity. Although one would suspect that this could lead to some confusion, it rarely does because the context in which rho is used will usually define whether it is representing density or resistivity unambiguously. In these notes, we will follow standard geophysical practice and use rho to represent both physical properties.
*Unfortunately also WWW conventions make it difficult to display the appropriate Greek letter rho, as seen in the left hand side of the resistivity equation in the graphic above. If you see ρ in the text (or any other &###; construct) you are viewing the translation of an exotic symbol.
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