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Transactions. 



at intervals hot saturated solutions of brine up to 20 gallons, testing between 

 successive charges, and also subsequently. In pipes D cold saturated 

 solutions (up to 9 gallons) of copper sulphate were used. 



Table I. — ^Test Pipes, Bealey Corner. 

 (Average Individual Pipe-resistances to Earth.) 



Particulars of Pipes, 6 Chains apart. 



Pipes as 

 driven. 



Imme- 



d lately 



after 



salting. 



Seven 



Days 



after 



salting. 



I Twenty- Three I Seven 



one Days] Months I Months 



after after 1 after 



salting, salting, j salting. 



A. Three 2 in. pipos, 7 ft. deep 



B. Turee IJ in. pipes, 9 ft. deep 



C. Two IJ in. pipes, 9 ft. deep 



D. Two li in. pipe.s, 7 ft. 6 in. deep 



(salted witli copper sulphate) 



Ohms. 

 45 

 70 

 46 

 94 



These are all individual pipe- resistances obtained with a low- voltage D.C. bridge 

 method. 



Some discussion is here necessary concerning the various methods of test 

 that have been used. The low-voltage D.C. Wheatstone bridge method 

 has been most commonly used, owing to its ease of application and the 

 portable nature of the apparatus. This test cannot give the true resistance 

 under working-conditions, when thousands of volts A.C. are concerned, 

 but gives a proportional result, as indicated by the following observations : 

 Some earths at Addington Substation were tested by three different methods 

 — -{a) By D.C. bridge ; {b) by voltmeter-ammeter D.C. 110 volts ; (c) by 

 Wheatstone bridge, using surging A.C. voltage and telephone-receiver. 



In reference to (a) and (6), in all tests two values were obtained with 

 reversed polarity, to eliminate as far as possible electrolytic effect, and the 

 tabulated values are the mean of the two. Test (c) was efiected by placing 

 an interrupter (a small buzzer) in the primary of a small transformer, with 

 a 4-volt cell as source of e.m.f., and connecting the secondary to the 

 " battery " terminals of the bridge. A telephone-receiver replaces the 

 galvanometer for indicating a balance of the arms. The results are tabu- 

 lated below for three earths marked A, D, and P. 



Although the tests are not numerous, they are sufficient to indicate a 

 large but fairly constant error in the ordinary low-pressure D.C. bridge 

 test, and a fairly close approximation of the A.C. bridge test to the positive 

 voltammeter determination. , 



Iu the same connection Mr. Creighton may be quoted — " that this 

 method (A.C. bridge) gives values within 5 per cent, of those by D.C. or 

 A.C. voltammeter." 



