The curves of Fig. 11 are calculated from this equation for electrodes of 13, 25 and 100 mm diameter respectively in a soil of l00Ω.m respectively. Change of diameter has a relatively minor effect and size of pipe is generally governed by resistance to bending or splitting. It is apparent that the resistance diminishes rapidly with the first few feet of driving, but less so at depths greater than 2 to 3 m in soil of uniform resistivity. A number of rods or pipes may be connected in parallel and the resistance is then practically proportional to the reciprocal of the number employed so long as each is situated outside the resistance area of any other. In practice, this is satisfied by a mutual separation equal to the driven depth. Little is to be gained by separation beyond twice the driven depth. A substantial gain is effected even at 2 m separation. Pipes may be of cast iron of not less than 100 mm diameter, 2·5 to 3 m long and 13 mm thick. Such pipes cannot be driven satisfactorily and may, therefore, be more expensive to instal than plates for the same effective area. Alternatively, mild steel water-pipes of 38 to 50 mm diameter are sometimes employed. These can be driven but are less durable than copper rods. We, at Atul Casting and Metal Works manufactures all earthing related products such as G.I Strips, G.I plates, G.I Rods, G.I Pipes, C.I covers, Cable Trays, G.I Wire, Manhole Covers and Route Markers for past 50 + Years in heart of New Delhi. Call us at +91 9873361672 for best deals