470 Transactions. 



Thus resistances by D.C. measurement are all higher than those by 

 A.C. As the resistance is — at least, very largely — the resistance of an 

 electrolyte, this is to be expected by reason of the polf.rization efiect with 

 D.C. ObAaously, values by voltmeter-ammeter methods must be accepted 

 as correct, and whether D.C. or A.C. determinations should be considered 

 in any specific case will depend on working-conditions. 



The A.C. bridge was found both quicker and more convenient in opera- 

 tion than the D.C. bridge. A galvanometer, however well packed, will 

 not stand continuous transport by motor bicycle without losing in sensi- 

 tiveness, while a telephone-receiver is easily packed, and with reasonable 

 care does not suffer from carriage. 



A very compact form for the transformer and interrupter can be 

 obtained by rewinding the electro-magnet of an ordinary 4-ohm buzzer, 

 and using one leg as primary and for working the interrupter, and winding 

 a secondary on the other leg. Eight layers of No. 36 copper on the primary, 

 and eleven layers of same gauge on secondary, gave very good results. 



Some comment is necessary relative to the practice of solving simul- 

 taneous equations to obtain the resistances of indi\adual earths. The solu- 

 tions obtained will be correct as long as the unknowns are independent 

 quantities — i.e., as long as one earth does not interfere with another. This 

 depends mainly on the distance between the earths being greater than some 

 critical value, which, again, is dependent on the class of soil. Wliere indi- 

 vidual resistances are tabulated herein, unless otherwise stated, the earths 

 are sufficiently far apart not to interfere with each other in any way. 



It became evident at once that in the class of ground encountered earth- 

 resistances below 100 ohms were going to be very difficult and expensive 

 in practice. 



From the tests made it appears that once a pipe reaches a depth of 

 6 ft. to 7 ft., increase in depth has little efiect on the final value. Any 

 improvement produced by increased depth is so small as to be lost in the 

 effect of other factors. It is common experience that in considering ground 

 connections the character of the soil is the main factor concerned. How- 

 ever, it is not the solids constituting the soil so much as the moisture and 

 the salts in solution which command attention. Messrs. McCollum and 

 Loffan (Proc. A.I.E.E., June, 1913) pubhsh some material bearing on the 

 conductivity of soils with varying moisture-content (diagram No. 1). The 

 curve shown was obtained from a sample of red-clay soil which had been 

 dried out at 105° F., and water afterwards added. It will be noticed 

 that the resistance is practically constant for a content greater than 20 per 

 cent. In the same paper are recorded ninety-two values of specific resist- 

 ances of " a wide variety of different kinds of soil." In 50 per cent, of the 

 determinations the moisture-content, which is also recorded, lies between 

 19 and 30 per cent. — i.e., they were moist soils — and at these values the 

 resistance-variation with moisture-content is almost negligible. Yet the 

 recorded specific resistances range from 41,490 to 470 ohms. In some 

 (jases— few rather than many — the quantity of moisture is distinctly respon- 

 sible for the variation in resistance ; but in general the main factor aft'ect- 

 ing the conductivity of the pipe earth is the electrolytic quality of this 

 moisture. 



Considering the exceedingly high resistances of both approximately dry 

 soil and approximately pure water, we must conclude that the conductivity 

 — at least, in the immediate neighbourhood of the earth connection — is 

 almost solely due to the salts, alkalies, or acids in solution. The exceed- 



