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MILHDBK419A
2.6 RESISTANCE PROPERTIES.
2.6.1 Simple Isolated Electrodes.
2.6.1.1 Driven Rod. The resistance to earth of the vertical rod in homogeneous earth can be developed by
approximating the rod as a series of buried spherical elements (23). When the contributions of the elemental
spheres are integrated along the length of rod and its image, the resistance to earth of the vertical rod is
computed to be:
where
d = rod diameter, in cm,
= earth resistivity in ohmcm,
= rod length, in cm.
An inaccuracy in the derived result arises from the assumption that equal incremental currents flow from the
incremental spheres. Actually, more current per unit length flows into the soil near the earth's surface than at
the lower end of the rod. It has been found empirically that the expression
is a better approximation to the resistance to ground for a driven vertical rod. The net difference in resistance
as given by Equations 215 and 216 is about 10 percent.
The resistance of the rod is directly affected by changes in the length of the rod and by the logarithm of the
length Changes in the diameter only show up as slight changes in the logarithm in Equation 215 and 216.
Figures 26 and 27 show the measured changes in resistance that occurs with rod length and rod diameter. It
is evident that effects of rod length do predominate over the effects of rod diameter.
217


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