466 Transactions. 



to the city water-main. In July, 1915, tests of these earths by the authors 

 gave the following results : Plate 1 to plate 2=6 ohms ; plate 1 to water- 

 main = 4 ohms ; plate 2 to water-main = 5 ohms. 



During the course of the present investigation plates were tried on the 

 transmission-line. Three copper plates 12 in. square were buried at a depth 

 of 5 ft. 6 in. to 6 ft., and bedded in clay. In one case the country was 

 loam to shingle, w^hile the. others were clay to sand. The plates were placed 

 at intervals of 6 chains, and tests of pairs in series gave the following 

 values : Eesistance of plates immediately after placing — No. 760 plate to 

 No. 762 plate = 260 ohms ; No. 759 plate to No. 760 plate = 200 ohms ; 

 No. 759 plate to No. 760 plate = 180 after 4 gallons water had been poured 

 over each ; No. 760 plate to No. 762 plate = 130 after 3 gallons of brine 

 had been poured over No. 762 plate. Values by X.Q. bridge test. 



The contrast of these values with those quoted is obvious, and these are 

 from by no means in the worst districts. This chiefly indicates the un- 

 reliability of the earth connection by means of small copper plates. At 

 the same time, since each earth plate in place costs from £1 to £1 10s., the 

 cost for such inadequate result precludes the adoption of the plate earth on 

 any extended scale. 



At the erection of the transmission-line poles provision was made for 

 earthing by carrying a strip of galvanized hoop iron (IJin. by -^^ in.) down 

 the pole and carried round the butt three times below ground-level, ter- 

 minating underneath the bottom of the pole. The connection was thus buried 

 at a depth of at lea-^t 6jt. No precautions in the way of selecting earth 

 to make the contact were taken when filling in, and at bends and in special 

 cases the poles were concreted. Subsequently tests of these earths were 

 made with an instrument consisting of an ordinary Wheatstone bridge 

 and 4-volt dry cell. 



To get the resistance of each separate earth connection, the strips were 

 tested in groups of three throughout the 1,737 poles, and the values computed 

 from the three simultaneous equations obtained. (Notes concerning both 

 the test and the method of computation of individual values will be found 

 later.) 



The values obtained ranged from 10 ohms to 5,000 ohms for indi\ddual 

 connections. In some districts the resistances were consistently low ; in 

 others consistently high ; while still others gave resistances indiscriminately 

 high and low. Of the 864 earths on the north Une, more than 30 per cent, 

 were over 1,000 ohms, and 83 per cent, over 100 ohms ; while on th*e south 

 line, out of 873 poles, 18 per cent, were over 1,000 ohms, and 90 per cent, 

 over 100 ohms. As earths of this order are quite useless for practical pur- 

 poses, attention was turned to pipe earths. 



Bv far the most useful data on the subject available consists of a paper 

 by E. E. F. Creighton in the " General Electric Ke\iew," vol. 15, page 66, 

 Februarv, 1912. The most important and pertinent conclusions are here 

 quoted : — 



" 1. Resistance of a pipe earth varies inversely as the depth of the pipe 

 after the pipe has reached a uniformly conducting stratum. 



" 2. Practically all the resistance is in the earth in the immediate vicinity 

 of the pipe. This resistance depends on the specific resistance of the material. 

 The specific resistance depends on acids, salts, or alkahes in solution about 

 the plate. To get the lowest possible resistance strong salt water should be 

 poured around the pipe. The chemical action of salt on the iron of the pipe 

 under average conditions is negligible. 



