ELECTRICAL METHODS 529 



personnel, the use of this electrode is almost as rapid as that of the 

 mobile electrode. 



Insulated Wire for Field Use. — A stranded conductor covered with 

 bare rubber insulation is preferred. Cloth braid and other protective 

 coatings for the rubber have not proved satisfactory, principally because 

 the cloth abrades very fast when the wire is drawn across rocks and 

 through brush. In addition, when cloth braid is used, it is difficult to 

 detect breaks in the conductor. 



A single conductor of No. 16 American wire gauge is satisfactory 

 for short lines. One wire of this size on the market consists of 13 strands 

 of hard drawn copper, each of which has a diameter of 0.0142 inches and 

 is covered with a thirty per cent rubber insulation of approximately 3/64 

 inch thickness. For the longer high voltage energizing circuits in which 

 the voltage may rise to 2000 volts or more, it is recommended that a 

 single conductor No. 14 American wire gauge, consisting of 19 strands 

 of 0.0142 inch hard drawn copper with 5/64 inch thirty per cent rubber 

 insulation, be employed. This wire has an estimated tensile strength of 

 approximately 190 pounds. The estimated break-down voltage for the 

 5/64 rubber insulation is 24,000 volts, while for the 3/64 inch rubber 

 insulation it is approximately 15,000 volts. 



During field work in which the conductor is subjected to considerable 

 strain, a single conductor having twelve strands of 0.0126 inch tinned 

 steel and 14 strands of 0.0126 inch tinned copper, bunch stranded with 

 2-inch pitch, has proved highly satisfactory. This conductor is covered 

 with a 5/64 inch thick, thirty per cent rubber insulation and has a tensile 

 strength of 350 to 400 pounds. 



In dry areas where potentials of less than 100 volts are used, standard 

 flexible fixture wire may be employed for the potential and energizing 

 circuit leads. Fixture wire is usually No. 18 or No. 16 gauge stranded 

 copper, covered with 1/64 inch rubber composition insulation and a cotton 

 weatherproof, pitch-impregnated braid. This wire will stand a tensile 

 pull of approximately thirty to fifty pounds. 



Wire Splices 



Several precautions should be observed when making splices for 

 repairing breaks in the line wire. A layer of self-vulcanizing rubber 

 tape, covered with a layer of friction tape, should be employed. The 

 usual width of friction tape is approximately three-quarters of an inch. 

 It is advisable to split the tape to a width of approximately one-fourth to 

 three-eighths of an inch. Prior to making measurements, all splices 

 and field wire should be tested for line leakage. This can usually be 

 accomplished in the field by disconnecting the ends of the wire from their 

 respective electrodes and applying an overload voltage to the lines. 



