 Chapter 6 |
 Contents |
 Chapter 8 |
133. In order to maintain uninterrupted telegraphic communication between any two points, it is of the first importance that the line should be well constructed and properly insulated throughout. There are numerous minor details in the construction and repairing of telegraph lines which merit much more attention than they generally receive. The bad working of our lines is in a great measure owing to the neglect of these apparently trifling details, through ignorance or carelessness.
134. Poles.--- The poles intended for an ordinary line should never be less than five inches in diameter at the top, their length depending upon the number of wires to be provided for, and in some measure upon the location of the line. They should be set in the ground to the depth of five feet, wherever practicable.
In setting poles around the curve of a railway, they should be made to lean back against the strain of the curve.
135. Wire.--- For ordinary lines, galvanized iron wire, of No. 8 or 9, Birmingham gauge, is generally employed. For short lines, No. 10 or 11 will answer very well. The ``American Compound Wire,'' a recent invention, is composed of a combination of a steel core with a sheathing of copper. It has come into extensive use within the short time which has elapsed since its introduction, and has, thus far, been found to answer admirably. A wire of this kind, having a conductivity equal to a No. 8 iron wire, weighs but 112 pounds per mile.
136. The less the size, and consequently the conductivity of the line wire, the more care is required in its insulation, for an increased resistance virtually adds to the length of the circuit. Increased conductivity thus admits of a reduction in battery power, with a consequent decrease in the escape of electricity, and long circuits may be thus worked with much greater facility ; a fact which has been most unaccountably ignored in this country.
137. Galvanized or Zinc Coated Wire must always be used for permanent work, for rust reduces the conducting power of wires very rapidly. This is especially the case with the smaller sizes, such as No. 11 or 12. In smoky places it is a good plan to paint the wire before it is put up, for the gas arising from the combustion of coal destroys the zinc coating in a short time, as may be observed in many of our larger cities.
138. Arrangement of Wires upon the Pole.--- Wires arranged vertically upon the poles, or one above another are more liable to get into contact with each other than when arranged horizontally upon cross-arms. When placed one above another, each alternate wire should be fastened upon opposite sides of the poles.
It is better not to place wires of different sizes upon the same poles or cross-arms if it can be avoided, as they are much more likely to get ``crossed'' than wires of the same size would be, as they do not keep time with each other when swung to and fro by the wind.
139. Joints or Splices.--- In the construction of a line nothing is of greater importance than the perfect continuity of the circuit, and this depends, in a great measure, upon the perfection of the joints. The importance of this has been very generally overlooked by the telegraphers of this country, and much trouble in working lines has been experienced in consequence, the cause of which has remained unsuspected. A single rusty unsoldered joint will often cause more resistance than fifty miles of line.
No joint or splice, however clean and firm, can be depended upon if made by mere contact or twisting. Sooner or later the metals will certainly rust, and this tendency is increased by the passage of the current. When copper and iron wires are joined together the joint is especially liable to become defective from this cause. It is a common error to suppose that joints made in galvanized wire do not require soldering.
140. In making a joint each wire should be twisted round the other, in the manner represented in Fig. 55, the turns passing as close, and as nearly at right angles as possible to the wire which they surround. A wire must never be spliced by being bent back and twisted around itself.
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141. The best solution for soldering is chloride of zinc, with a little muriatic acid added, for the purpose of cleansing the wire. In connecting copper and iron wire together, it is well to wash off the chloride of zinc, and then coat the joint with paint or rosin, or else to solder with the rosin alone. This will prevent local galvanic action between the metals.
142. Fixing the Insulators.--- In attaching insulators to the poles they should be arranged in such a manner as to prevent, as far as possible, the lodgement of snow about them, so as to form an escape between the wire and its support. The glass insulator is usually cemented to the bracket by means of white lead or asphaltum. The edge of the insulator must never be permitted to touch the shoulder of the bracket ; for in this case, during a shower, a continuous stream of water flows directly from the wire to the pole, entirely destroying the usefulness of the insulator. For the same reason an insulator ought never to be fastened down to a bracket by means of a spike driven over it, as is often done where there is an upward strain upon the wire. The proper way, in such cases, is to use some form of hook, or suspension insulator, and fasten the line into it with a tie- wire.
In turning a sharp angle it is better to put on two insulators and brackets at the corner pole, or the wire will be liable to come in contact with it.
When the Lefferts or Brooks insulator is used, there is danger of fracturing the glass while stringing wire, by violently wrenching the wire into the hooks. By a little precaution this result may be avoided.
143. Insulators and brackets are sometimes attached to a cross-arm, or other support, in a horizontal position. This ought never to be allowed, for a driving rain will wet the whole inner surface of the insulator, causing a great leakage of the current at every support. The same thing often occurs with improperly shaped brackets, which cause the spray from falling rain-drops to be dashed against the inside of the insulator. The shoulder of the bracket ought to be rounded or sloped off, so as to prevent this from happening.
Unless the insulator is securely fastened to the pin or bracket which supports it, it is liable to be lifted off by the wind, causing an interruption.
144. Leading Wires into Offices.--- The wires leading into offices are fruitful sources of escapes and other interruptions, as the work is often very unskilfully or carelessly done. Gutta-percha covered wires, unless well protected, become entirely useless in a year or two, if exposed to the air and light. The method employed in England to protect this kind of wire might be adopted with great advantage in this country. The gutta-percha wire is first covered with tape, and then saturated with a preservative mixture.*
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* This mixture is made and applied as follows: Take equal portions of wood tar, gas tar and slacked lime. Boil these together, stirring them well while boiling, until the moisture is entirely driven out, which may be known by the subsidence of the frothing. When cool apply to the taped wire, and then cover the latter with dry sand. Hang the wire up to dry in the air, and in three or four days it will be ready for use. This coating resists sun and moisture, and effectually protects the gutta- percha.
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The best way to lead wires through the side of a building is to enclose them in hard rubber tubes, with the outer ends inclined downwards, to prevent moisture from entering. In arranging these wires, it should be borne in mind that the current will follow moisture and dampness along the outer surface of covered wire, unless it is so placed that the line of leakage is broken at some point.
145. Fitting up Offices.--- In running wires inside an office, it is better never to allow two wires to touch each other, even when covered with an insulating coating, as this may be burned by lightning or otherwise rendered imperfect causing a cross-connection. The proper mode of arranging the office connections and running the wires to the instruments is shown in Figs. 17 and 18, pages 34 and 35. Splices in the office wires should be avoided as far as possible, but when required, they should be made by turning each wire eight or ten times around the other. A less number of turns answers for the line wire, because the strain tends to keep the joint pressed together. Great care must be observed in making the joint between the iron and copper wire, which must in all cases be soldered.
146. Ground Connections.--- It is of the utmost importance that the ground plate at each end of the line should make a perfect connection with the earth. The plate must be large, and buried deep in wet soil below the reach of frost. A water or gas pipe makes an excellent ground connection. The ground wire should be attached outside the metre, as the latter is liable to be occasionally disconnected for repairs. It is advisable, whenever practicable, to form a connection both with the gas and water pipes. The connection should be carefully made and always well soldered.
147. Cables.--- The shore ends of cables should be bedded well out to low water mark. Dig the trench to a good depth, and cover the cable with a piece of heavy plank or joist, and secure it well with heavy stones, laid at short intervals. If the covering be merely of sand it will soon wash away and leave the cable uncovered. Never allow any portion of a cable to be exposed to sun or air, but cover it all the way from the box where the connection is made with the air line.
Cable boxes always ought to be made double (one box within another), in order to prevent wet from entering. The unskilful manner in which these are often arranged is a fruitful source of trouble in working lines.
Lightning arresters should be kept attached to cables all the year round. It is not uncommon for heavy lightning to occur in midwinter in this country.
148. Making Joints in Cables.--- In splicing cables, or other gutta-percha wire, the following is the method recommended by the Bishop Gutta-Percha Company, who have manufactured the greater portion of the submarine cables in use in this country :
``Use gutta-percha one sixteenth of an inch thick, cut in pieces to suit the joint. Soften it in hot water, and keep it flat. Wipe the surface with a cloth. Heat the surface by holding it near a flat file or other iron, about as hot as a laundress's iron ; if the iron causes the gutta-percha to smoke, it is too hot. When dry and a little sticky, wind two or three coatings of gutta-percha around the joint, taking care that each coating is perfect and each layer is dry ; then smooth off and lap the joint well over on the gutta-percha on each side of the joining. Use no spirit lamp, nor anything with a blaze. When gutta-percha is burned it cannot be restored. Hot water joints are worthless. They will not stand, and will open when dried out.
``In making joints it is absolutely necessary that the hands of the operator should be clean, and that no water, grease, dirt, or anything of the sort must be allowed to touch the gutta-percha.''
149. Another method is given in Culley's Handbook of Practical Telegraphy, as follows :
``Prior to making the joint the gutta-percha is removed from the ends of the wires for about one and a half inches, and the copper wires are carefully cleaned by scraping ; the wires are twisted together for one inch, the sharp ends being closely trimmed off. The joint is then soldered with rosin and good soft solder, containing a sufficiency of tin.
``After this the gutta-percha is scraped, or very carefully pared back for about two inches, to remove its outer surface, which is oxidized, and will not join properly ; the wire joint is covered with Chatterton's compound* and the gutta-percha, heated on both sides, and tapered down over the joint till that from each side meets. The junction is completed by means of a warm joining tool, care being taken to mix the gutta-percha well without burning. As soon as this has cooled another coating of Chatterton's compound is spread over the gutta- percha, taking care not to burn the compound.
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* The ingredients of this are by weight, as follows one part of Stockholm tar; one part of rosin, and three parts of gutta- percha.
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``A new and clean sheet of gutta-percha is then heated by means of a spirit lamp, and while so heated carefully stretched so as slightly to thin it. Then, while it and the Chatterton coated joint are still hot, it is laid on the joint, and pinched tightly round it with the finger and thumb, after which it is trimmed off close with scissors. The seam is again pinched and carefully finished off with a warm tool, so as to mix the gutta- percha of the two sides, and the coating of the wire itself, well together.
``The joint, when cool, is again covered with Chatterton's compound, and a longer and larger sheet of gutta-percha is laid over it, pinched, cut, and tooled off as before.
``When the joint is complete, another coating of Chatterton's compound is applied over the whole, well tooled over the joint, and when cool, rubbed with the hand, well moistened, till the surface is smooth.
``The mixing of the old and the new gutta-percha is most important, and joints generally fail from this having been imperfectly done, or from the percha being overheated. Cleanliness is essential to success. The fingers should be used as little as possible, and must be kept very clean.''