Publication: The Electrician & Electrical Engineer
New York, NY, United States
THE CONSTRUCTION OF LINES FOR ELECTRIC
BY THOMAS D. LOCKWOOD.
(Continued from page 242.)
Turning now to the subject of insulation, it is to be noted that in addition to the well-known insulating materials, such as india rubber, gutta percha and kerite, a variety of new substances have recently been introduced: these are ozokerite, various forms of petroleum residuum, and bitite, which is practically vulcanized bitumen. There is really very little to say about any of these. We do not advocate the universal use of gutta percha, although there seems to be nothing so satisfactory for subaqueous cables. Its strong affinity for oxygen renders it almost useless for open air work as it cracks very soon and the insulation is then ruined ; gutta percha whenever used in the air should be covered with tape soaked with Stockholm tar.
India rubber is a first-class insulator — and has a lower inductive capacity than gutta percha, and as compared with gutta percha it is for atrial cables a great improvement. Its defects are that it can absorb water, and so tends in some degree to lower its insulation after it has been used some time, and also that when placed close to copper wire it decomposes, forming a viscid, tarlike substance. If conductors can be secured, in which the wire is exactly in the centre of the rubber insulation — the group of wires being well insulated and protected from injury and the weather by some external and reliable envelope — and if the precaution is taken of covering each copper wire with a layer of cotton before the rubber is put on, which proceeding prevents the chemical change referred to, there seems to be no reason why rubber should not prove to be a very satisfactory insulator.
Kerite is a vulcanized compound of asphaltum or tar mixed with certain vegetable oils. It is a good insulator, has a reasonably low inductive capacity, and if properly protected is very durable. Electrically it is first-class, and wherever kerite cables are used they are appreciated.
The new insulating compounds which have been referred to, are highly commended by their proprietors, are well spoken of by experts, and to the knowledge of the writer are very high insulators. The one element of durability is with them still uncertain, at least so far as our personal knowledge is concerned. We have not proved them.
There remains still one cable to be noticed, viz., the paraffin cable. Paraffin melts at from 113° F. to 149° F., and when solid is one of the best insulators known. Its resistance, however, depreciates very rapidly with warmth. It is obtained from coal tar, bituminous shale, peat, and mineral oil. One of the best telephonic cables now made consists of a group of copper wires each covered with two or more windings of cotton saturated with paraffin, and encased in a lead pipe. The space between the wires and the pipe is filled with paraffin, and the entire filling is heavily charged with gas which is scattered in globules throughout the mass. The electrostatic capacity of this cable is very low, and experience has demonstrated that it is a serviceable cable.
It is then evident from the foregoing considerations, that in ordering cables for aerial work, paraffin, rubber, and kerite are the materials which have proved themselves to be adapted for the purpose of maintaining insulation, and if either are chosen, they will probably prove satisfactory if well put together mechanically. For protection, the paraffin cables as made by the Western Electric Co., are, as already indicated, enclosed in lead piping; while the kerite cables are wrapped with kerite tape, and the rubber cables have been enclosed in painted canvass. There is, however, much still to be desired in this respect, and a protective envelope for osrial cables that is light, strong, durable, and capable of successfully withstanding for a long time the action of wind and storm would be very acceptable.
We now approach the different means which have been adopted to counteract or neutralize induction between wire and wire.
Until the advent of telephony, this obtrusive element in the electric transmission of intelligence was not often troublesome in the case of overhead lines. In telephonic transmission, however, it is very annoying indeed. Whether in bare wires supported as usual on insulators and poles, or in cables as we have described, this evil appears. Many of the disturbing currents known to the average telephonic employe or to the general public as induction, are really due to other causes. These are briefly: earth currents, atmospheric electricity, thermo electric currents arising from the ever varying thermal conditions of the earth and air, and magneto electric currents due to the swing of long wires, when stretching in an east and west direction, across the magnetic meridian of the earth. With disturbing currents due to these influences we have at present nothing to do. With all this, there are yet a multitude of disturbing effects produced by actual induction between wire and wire, resulting, if some of the wires are used for telegraphs and some for telephones, in the fact that the telegraphic signals are heard in the telephones, sometimes so loudly as to drown the legitimate sounds of articulate speech which are desired. If all the wires are used in connection with telephones, the message sent on one wire may be heard in the telephones connected with the others. What shall we do to prevent this ? It is a fact worth knowing that in a cable of any given number of wires, say 50, if we speak on any one wire, the words can be heard nearly as well in a receiver connected with another wire, provided the remainder of the wires in the cable are left disconnected or open; but if we connect all the others to ground, the effect is greatly reduced, and practically dissipated. This leads to the conclusion that if we have a cable of more wires than are necessary for our business, by grounding or by looping the spares together at the ends, we may dispense with any other antiinduction devices.
For cables which are less than 500 feet no preventive of induction seems to be absolutely necessary, and half a dozen extra wires, grounded at both ends, have sufficed.
For longer cables it has for some years been customary to adopt the expedient patented by Dr. Foucault, and to surround each insulated conductor with a metallic coating (usually tinfoil) in connection by wires at suitable points with the earth. This remedy certainly subdues not only the induced currents, but also the extraneous currents due to other causes which have been referred to, and enables reasonable work to be got out of the cables. It is true that it also subdues the articulate speech, or rather the currents which reproduce the same, but not in so great a degree; and it has thus proved to be a great assistance to cable construction. It is thought that it would be still more efficient if a thicker metallic coating could economically be employed, or if iron could be commonly used for the outside coating, as, in the first case, the conductivity of the metal sheath would be greatly improved as a circulating medium for the induced currents, and in the second place, the induced electricity is transformed into magnetism in the iron coating. It has also been proposed to insulate the outside metallic coating of each wire, and then to loop them together in pairs at each end; but it is obvious that this is substantially the same expedient which was at first mentioned. In the paraffin cables which we have described, the only remedy against induction is a large central wire which at the ends may be connected either with the ground or with the enclosing tube, each wire apparently performing the function of an induction screen for the others. The most effectual arrangement for neutralizing induction is, however, to dispense with the use of the ground as a return circuit altogether, and to provide a metallic return wire in lieu thereof, the two wires being so placed as to be equidistant from the disturbing wires. This is best accomplished by twisting the two wires of each circuit together. This arrangement has not greatly found favor in the eyes of exchange managers, since it involves the use of a double wire not only for each conductor in the cable itself, which would be a small matter, but also throughout, in the remainder of each line, and in the switch apparatus also. The necessity for employing anti-induction devices limits the length of cable which may profitably be employed, because the devices which have proved to be most generally useful — i.e.,those which bring the earth into close proximity to the outside of the insulated conductor by means of a grounded metallic envelope, unfortunately increase the electrostatic capacity of each line materially, and thus cause a very pronounced degree of electrical retardation if the cable exceeds in length a maximum of two miles.
(To be continued.)
|Date completed:||January 18, 2011 by: Bob Stahr;|