BiRKS AND Webb. — Resistance of Earth Connections. 



477 



copper. These tested against independent earths (either water or gas 

 mains) gave the following results (A.C. bridge method) : — 



Table VII. — Christchurch City Council Earths. 



* Since the assumption is that values are independent quantities, and in a city 

 pipes run in unknown directions, some of the values will be slightly in error, as in tliis 

 case, owing to the unknowns of the simultaneous equations being in some cases not 

 entirely independent. Hence the above negative values. 



At the transformers, the pipe earths and plate earths were placed within 

 a few feet of each other, and tests show in favour of the pipe earth. 

 As might be expected, in good conducting soil there is very little difiererce 

 between plate and pipe earths, but in high-resistance soil the salted pipe 

 usually has a considerable advantage. 



The practice pursued by the Public Works Department (diagram No. 7) 

 in the general transmission and distribution earths is to drive a 1 in. pipe, 

 drilled with small holes every foot, to a depth of 6 ft. to 8 ft. in ground ; 

 then to pour 4 gallons of concentrated brine down the pipe, and fill the 

 pipe with rock salt, placing some rock salt round the top. The top of the 

 pipe is stopped with a wooden plug, which can easily be removed for re- 

 plenishing the supply of salt inside the pipe. The conductor consists of 

 1 in. galvanized-iron strip, riveted round the pipe and bolted through. In 

 attaching the lead it is essential to make a good connection mechanically. 

 Also any combination of metals which would yield to electrolysis should 

 be avoided, or othei-wise the joint must be thoroiighly covered with some 

 waterproof material. On this account, as well as for economy in cost, 

 the joint is not sweated. 



