A NE]V TYPE OF UNDERGROUND TELEPHONE WIRE 453 



One consideration of some inii)or(ance in installinji wire of this type 

 is the possibility of the insulation being crushed by boulders displaced 

 b\ the ])l()\v, ])articular]y where the trench with wire in ])lacc is to be 

 rolled down or subjected to heavy tratttc. This danger is, in fact, of 

 such importance that buried wire of this type is probably not a service- 

 able form of construction through a terrain where nested boulders are 

 frec]uenth' encountered. 



While it is generally possible to plow across gravel highways, this 

 method can not be used when hard-surface highways are encountered, 

 and in such cases it becomes necessary either to use a pipe pushed under 

 the roadway or to span the highway with open wire. Where conditions 

 are such as to require routing the wire through or over culverts, across 

 ditches, streams and the like, involving actual or potential exposure 

 of the wire as by soil erosion, iron pipe or equivalent protection against 

 mechanical injury and light will generally be required. 



Interference 



As in the case of other types of telephone circuits; the problem of 

 avoiding noise and crosstalk must be considered. Where more than 

 one pair is buried at a time, there is a crosstalk problem but experience 

 has shown that the introduction of twists every few feet, either by 

 twisting the wire in the process of laying or by having it pretwisted 

 on the reels, is sufficient to reduce the crosstalk to low values. 



Special care must be given the wire in manufacture to assure a good 

 degree of balance between the capacitances of the two conductors to 

 ground. This is important in order to avoid noise in the buried wire 

 circuits when they are exposed to power circuits or when the connected 

 open wire is exposed. Under severe exposure conditions, even with 

 the best balance obtainable in manufacture, it may be necessary to 

 resort to special balancing measures in the field to assure satisfactorily 

 quiet circuits. 



Maintenance Questions 



The introduction of a new type of plant such as buried wire will 

 naturally involve some new maintenance problems. Although records 

 will, in general, be kept of buried wire routes, it will at times be desir- 

 able to have fairly precise methods of tracing the underground path. 

 Experiments have indicated that this may be done with considerable 

 precision by putting a tone current on the wire and following along the 

 surface of the ground with an exploring coil device. The location 

 of faults in buried wire also involves some problems which are different 

 from those experienced with cable circuits but experiments have indi- 

 cated that established methods may be adapted to this new use with 

 an acceptable degree of precision. 



