A

where again, L and A, respectively, are the length and cross-sectional area of the cylinder, and p is the resistivity.

Resistivity is a property only of the material composing the cylinder (Figure 5.1) and represents the capability of that particular material to oppose the flow of electric current. The p values for soil and rock materials are typically reported in units of ohm-meters (Am). Electrical conductivity, o, is the reciprocal of p (= 1/p), and is a property indicative of a material's ability to convey electrical current, not oppose it. Instead of p, the value of o, in units of millisiemens/meter (mS/m), is most commonly reported in agriculture. For reference, a p value of 1 Am corresponds to a o value of 1000 mS/m. Furthermore, it should be pointed out that agricultural literature often employs EC as the symbol for electrical conductivity rather than, o, which is the symbol for electrical conductivity routinely found in near-surface geophysics texts. The resistivity methods utilized for agricultural purposes are focused largely on the determination of o (EC) values for soil materials.

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