BiRKS AND Webb. — Resistance of Earth Connections. 



475 



that the minimum useful distance between two pipes in good ground is 

 about 6 ft. when the pipes are unsalted. After pipes have been salted the 

 minimum distance must be increased. 



In general, when pipes are salted, they must be placed at least 12 ft. 

 apart to retain 90 per cent, of the efficiency of each pipe. The actual distance 



necessary in any particular case depends on the conducti^'ity of the soil 

 in which the pipes are placed, and the higher the conductivity the farther 

 apart must the pipes be to retain their efficiency. 



Groups of pipes were placed on the transmission-line for the purpose of 

 earthing depot telephones and apparatus. The following table gives 

 particulars and results obtained with several pipes in parallel. No two 

 pipes were placed less than 12 ft. apart. 



Table VI. — ^Pipes in Parallel. (All A.C. Bridge.) 



Locality. 



Number 



of 

 Pipes. 



Average Resistance 

 of each Pipe. 



Unsalted. 



Salted. 



Resistance of all 

 Pipes in Parallel. 



Imme- 

 diately 

 after 

 salting. 



Five 

 Months 



after 

 saltings 



Class of Soil. 



Greendale . . 

 Charing Cross 



Aylesbury . . 

 West Melton 



Bealey 

 Glenroy 



Highfield .. 



Sandy knolls 



5 

 4 



7 

 4 



4 



4 



4 

 5 



Ohms. 

 1,900 



840 



2,000 

 910 



534 



180 



55f3 

 2,334 



1.50 

 94 



126 



417 



34 



26 



31 

 90 



61 



Very dry clay and shingle. 



Heavy shingle cemented with 

 clay. 



Heavy shingle. 



Sand and clay to heavy 

 shingle. 



Loam to heavy shingle. 



Loam and clay, heavy boul- 

 der, somewhat marshy. 



Loam and sand for 4 ft. to 

 shingle. 



Heavy running shingle. 



