### Resistance of Driven Rods

The most simple and cheap from of electrodes is nothing but driven rod. It is shown in the Fig. 1 with its length as l and diameter s with its image. It can be considered equivalent to an ellipsoid of revolution with a major axis very much large as compared to minor axis.
The capacity of rod and its image is
The earthing resistance of driven rod is given by
For the cylindrical rod with hemisphere and the above expression becomes,
An alternative expression for the resistance of driven rod is obtained by using uniform current disspipation method and is given by,

From the expression for resistance of driven rods, it can be seen that, the earthing resistance is inversely proportional to length l .
Generally resistance of single rod is not sufficient. Hence number of rods are connected in parallel. It is necessary to minimize the overlap among the areas of influence of the rods. This is possible by keeping large distance between the rods. It is difficult to achieve this in practice and approximate method is used.
In this method, a rod is replaced by hemispherical electrode having same resistance. If n rods are connected in parallel then their resistance is greater than (1/n) of that of a single driven rod due to the mutual screening.
The screening coefficient η for n electrodes in parallel is defined as,