BiRKS AND Webb. — Resistance of Earth Connections. 467 



" 3. If an iron pipe 1 in. in diameter is driven into normally moist earth 

 to a depth of 6 ft. to 8 ft. it wilhisually have a resistance of about 15 ohms- — 

 8 ohms mav be considered unusually low- — while in dry soils it may give 

 a resistance of 5.0 ohms and upwards. When it is desired to lower the resist- 

 ance to earth below that of a single-pipe earth, others should be driven at 

 distances of not less than 6 ft. apart. Then the total conductances will be 

 only slightly less than the sum of the conductances of the individual pipes. 



■' 4. Half the total resistance lies within 6 in. of the pipe. At a distance 

 apart of 6 ft. the resistance reaches nearly a constant value. 



"5. Since the resistance of a pipe earth lies mostly in the immediate 

 vicinity of the pipe, the greatest potential drop when the current flows 

 will also be concentrated there. Heating and drying out of the soil will tend 

 to magnify this efiect. The more salt water placed round a pipe earth 

 the less the potential gradient. 



" 6. The resistance of a pipe earth does not decrease in direct proportion 

 to the increase in diameter of the pipe. A pipe 2 in. in diameter has a 

 resistance only about 6 per cent, to 12 per cent, less than a pipe 1 in. in 

 diameter." 



Mr. H. P. Liversidge, before the International Association of Municipal 

 Electricians, 19th August, 1913, gives data recording resistances with pipe 

 earths (2 in. in diameter and 6 ft. to 12 ft. deep) ranging from 0-08 ohms 

 to 138 ohms. He states that the general average of some other twenty-five 

 difierent readings, representing fairly average conditions in reference to 

 character of soil, contained moisture, &c., give the following contact 

 resistances: Clay, 13-60 ohms; gravel, 6-01 ohms; top soil, 1-80 ohms. 

 There is no specific mention of the method of test employed, but apparently 

 a voltmeter-ammeter method, ^vith about 220 volts D.C., was used. All 

 these results were obtained after a salt solution had been poured round 

 the pipe, and probably these values are about one-half of the initial resistance. 

 Mr. Liversidge suggests as desirable values for an earth for station work 



1 to 2 ohms, and for other purposes 2 to 6 ohms. 



As it was apparently necessary to investigate in the actual country, 

 some preliminary pipe earths were then tried on the transmission-line at 

 Bealey Corner, where the line passes into the heavy shingle of the old Wai- 

 makariri river-bed. Galvanized-iron pipes 1| in. diameter were driven 

 4 ft. to 6 ft. deep beside line-poles, 6 chains apart. In sand and clay the 

 average individual resistance of seven pipes 5 ft. deep was 126 ohms by the 

 D.C. bridge method. In heavy shingle the average individual resistance 

 of four pipes 4 ft. deep was 1,800 ohms (D.C. bridge method). These pipes 

 were driven into the ground with a 14 lb. hammer, and no brine or other 

 electrolyte was added. 



Further pipes were then driven in the neighbourhood of three other poles. 

 The groimd about the three poles selected was loam for a foot at the top, 

 to clay and sand or clay and light shingle below. The imcertainty of the 

 substratum was demonstrated by the fact that, though one pipe was easily 

 driven in sand and clay, another, 6 ft. away, when driven a little more than 



2 ft. refused to go farther on account of heavy shingle. 



The pipes were drilled with two fin. holes at intervals of 1ft. along 

 the length of the pipe, and after starting with a crowbar were driven 5 ft. 

 to 8 ft. deep with a hammer. They were then tested as driven, and results 

 are tabulated below. Pipes A and B were tlien " salted " by filling with 

 salt, pouring water in, and filling again with salt. They were then tested 

 again, and at intervals subsequently. Pipes C were salted by pouring in 



