INSTALLATION OF FIXED EQUIPMENT 91 



a fully satisfactory ground is secured. This is conveniently done by using standard 

 emergency wire to make the test connections. Dry or frozen earth and rocks are 

 non-conductors and will not give a satisfactory ground. 



Nearly all grounding difficulties, however, arise from defective materials and 

 details of installation. Even after a suitable place is located, failure to satisfy the 

 primary requirements of good grounding, namely, a sufficiently large, clean, metallic 

 surface in intimate contact with permanently moist earth, will result in poor operation 

 of the station. 



2 -GROUNDING DEVICES 



There are several first-class grounding methods. Wherever possible the stan- 

 dard ground rod should be used. This is a 7-ft. galvanized-iron rod, | in. in diameter. 

 Drive the rod in so that only a length of about 8 in. remains above the ground surface. 

 Place as near the instrument as practicable and close up to the side of the building, 

 preferably where it will receive the drip from the eaves. If it is not possible to drive 

 the rod vertically to its full depth it should be driven 011 a slant, for it is essential that 

 the moist earth come in contact with the full length of the rod. Depth of freezing 

 must be carefully considered 1 , however, and if necessary to keep below frost line the 

 rod may be driven at the bottom of a hole. 



Wherever, for any reason, a standard ground rod is not available, a good ground 

 can be made on a copper plate or coil of copper wire. The plate should be about 

 No. 21 Birmingham sheet metal gauge or No. 21 U.S.S.M.G. in thickness and about 

 12 by 18 in. in size. A wire coil should be 12 in. in diameter and be composed of at 

 least ten turns o-f No. 12 bare copper wire. In the former case, a piece of bare copper 

 wire No. 12 or No. 14, must be soldered to the plate and should be sufficiently long 

 to reach to the surface of the ground. In the latter case, one end of the wire coil is 

 brought to the surface. The plate or the coil must be buried at a sufficient depth in 

 permanently moist earth, and the wire leading to the surface joined to the ground 

 wire of the instrument. 



A well may be used for a grounding place, but is of doubtful value if in rock. 

 Use a coil of copper wire as previously described "and, place flat on the bottom of the 

 well, having the end at least 3 ft. above the high-water level to provide for connection 

 to the wire from the instrument. 



When it is impracticable to find permanently moist earth, a good ground may be 

 obtained by placing a- similar coil of wire or a copper plate at the bottom of a hole 

 6 to 10 feet deep, covering it with about 1 ft. of powdered charcoal and wetting the 

 latter with a bucket or two of water. The hole should then be refilled and the earth 

 tamped in securely. Charcoal is hygroscopic, and will absorb and retain moisture. 

 The connection of the ground wire with the- coil must ~be soldered. A bucket of water 

 thrown on the ground at intervals during a dry period will add to its effectiveness. 



In some buildings it is possible to use a water-pipe for a ground, in which case 

 the ground wire should be attached to the pipe by a Blackburn ground clamp. Only 

 pipes which always contain water should be used. 



3 PRECAUTIONS 



Tinder no circumstances should ground wires be attached to : 



(1) Water-pipes of a town water supply if other lines are grounded on them. 



(2) Pipelines not carrying water. 



(3) Coils of iron wire or scraps of old iron thrown into a stream, lake or pond 

 having a rocky bottom. 



(4) Black iron plates, horseshoes, gun-barrels, ungalvanized bars, or pipes, rain- 

 spouts, old 'bolts, and similar articles. Ungalvanized iron placed in contact with the 

 earth rusts quickly. Rust is a non-conductor and acts as an insulator. Ground trouble 

 is sure to follow the use of the above articles. 



