480 Transactions. 



It still appears that in some districts single pipes will not provide a 

 reasonably low-resistance earth connection. In such districts it will be 

 necessary either to drive several pipes round each hole or to run a ground 

 wire along that portion of the line and connect it with the single pipe at each 

 pole. 



This discussion would not be complete without some reference to the 

 efficiency of the earth connection when called on to dissipate current. 



Concerning the behaviour of pipe earths under working -conditions 

 Mr. Creighton notes, '"With an applied potential of 1,000 volts the current 

 was so great that the earth round the pipe was quickly dried out, and 

 90 per cent, of the drop of potential took place within 1 ft. of the pipe 

 (unsalted). The pipe had lost its effectiveness as a ground. With 900 volts 

 drop in the immediate \dcinity, it was a dangerous condition."" 



The effect of salting on the potential drop has been illustrated by previous 

 curves (Nos. 3 and 3a). as the potential curve is of the same order as the 

 curve of resistance. Mr. Creighton pubhshes curves of potential distribution 

 which are very instructive, and further insists on the value of thorough 

 salting. With a potential difierence applied of 120 volts D.C. a drop of 

 70 volts took place in the first 6 in. unsalted, while the same drop was 

 extended over 3 ft, 6 in. after the pipe was salted. 



The capacity of an earth to discharge current over, an extended period 

 is also treated by Mr. Creighton : " The quantity of electricity that can be 

 passed through a pipe earth without materially changing its resistance 

 increases directly with the wetness of the earth in contact with the iron, 

 and the area of the iron surface exposed to the passage of current ; and 

 decreases as the resistance of the earth in contact with the pipe increases. 

 Certain critical values of the current may be carried continuously by a 

 pipe earth without varying the resistance. The higher the current above 

 this critical value, the more rapid the drying- out. To increase the ampere- 

 hour capacity, keep the pipe earth wet with salt water."" 



Curves are also published showing the advantage of the pipe earth over 

 a solid rod in both states, arid particularly its value when salted. A pipe earth 

 thoroughly salted carried a current of 56 amperes for a period of forty hours 

 without appreciable variation, while the same pipe earth initially carried 

 only 30 amperes, and was baked out to 4 amperes in four hours and 

 a half. 



Mr. Liversidge also made some tests, passing a current of 25 amperes 

 D.C. : " In most of the tests it was found that the pipe connection or 

 other grounding devices were able to carry a current of 25 amperes con- 

 tinuously without any marked increase in contact resistance. The results 

 which were obtained indicated in most cases that the limiting feature 

 of the current carrying-capacity of a ground connection is the ampere- 

 discharge per unit area of contact surface. If the current-density was 

 high enough to drive off the electrolytic moisture as a consequence of 

 excessive heating of the earth immediately surrounding the grounding 

 device, then the contact resistance would gradually increase." 



With alternating current it appears that the ampere-hour capacity of a 

 given earth is, in general, considerably higher than with D.C. This is 

 consequent on the excessive formation of films in the interspaces, conse- 

 quent on electrolysis, these films forming to a very much lesser degree, and 

 almost negligible, when the current is alternating. With a high frequency 

 the inductance of the earth and lead is a proportionally large factor for 

 consideration, and this must be reduced as far as possible by using shortest 



