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IIII 01111181 QI Ilk0 Imo". II' ' , M I ggllhiilhmlll !I Illllll horizontal
resistance and its electrostatic capacity are then measured. These tests are in some cases repeated at another temperature, say 5o F., for the purpose of obtaining at the same time greater certainty of the soundness of the core and the rate of variation of the conductor and dielectric resistances with temperature. The subjection of the P=h{w(A/sin i)f(uv cos i)} (a) core to a hydraulic pressure of four tons to the s9uare inch and an and w cos i = Bf (v sin i) (S), electric pressure of 5000 volts from an alternating-current trans- where f stand for " function." The factors Af (uv cos i) and former has been adopted, by one manufacturer at least, to secure Bf (v sin i) give the frictional resistance to sinking, per unit length the detection of masked faults which might develop themselves of the cable, in the direction of the length and transverse to the after submergence. Should these tests prove satisfactory the core length respectively). It is evident from equation (0) that the is served with jute yarn, coiled in water-tight tanks, and surrounded angle of immersion depends solely on the speed of the ship; hence with salt water. The insulation is again tested, and if no fault is in laying a cable on an irregular bottom it is of great importance discovered the served core is passed through the sheathing machine, that the speed should be sufficiently low. This may be illustrated and the iron sheath and the outer covering are 'laid on. As the very simply as follows: suppose a a (fig. lo) to be the surface of cable is sheathed it is stored in large water-tight tanks and kept the sea, b c the bottom, and c c the straight line made by the cable; at a nearly uniform temperature by means of water. then, if a hill H, which is at any part steeper than the inclination When the cable is to be laid it is transferred to a cable ship, of the cable, is passed over, the cable touches it at some point t provided with water-tight tanks similar to those used in the factory before it touches the part immediately below t, and if the friction Laying, for storing it. The tanks are nearly cylindrical in form between the cable and the ground is sufficient the cable will either and have a truncated cone
risk
The " slack indicator " of Messrs Siemens Brothers & Co. yields a continuous indication and record of the actual slack paid out. It consists of a and coiled towards the centre. The different coils are prevented long screw spindle, coupled by suitable gearing with the cable from adhering by a coating of whitewash, and the end of each nautical mile is carefully marked for future reference. After the cable has been again subjected to the proper electrical tests and found to be in perfect condition, the ship is taken to the place where the shore end is to be landed. A sufficient length of cable to reach the shore or the cable-house is paid overboard and coiled on a raft or rafts, or on the deck of a steam-launch, in order to be connected with the shore. The end is taken into the testing room in the cable-house and the conductor connected with the testing instruments, and, should the electrical tests continue satisfactory, the ship is put on the proper course and steams slowly ahead, paying out the cable over her stern. The cable must not be overstrained in the process of submersion, and must be paid out at the proper rate to give the requisite slack. This involves the introduction of machinery for measuring and controlling the speed at which it leaves the ship and for measuring the pull on the cable. The essential parts of this apparatus are shown in fig. 9. The lower end e of the cable in the tank T is taken to the testing room, so that continuous tests for electrical condition can be made. The upper end is passed over a guiding quadrant Q to a set of wheels or fixed quadrants I, 2, 3, . . . then to the paying-out drum P, from it to the dynamometer D, and finally to the stern pulley
to the piston P. The newly coated wire is passed through a long trough T, containing cold water, until it is sufficiently cold to allow it to be safely wound on a bobbin B' This operation completed, the wire is wound from the bobbin B' on to another, and at the same time carefully examined for air-holes or other flaws, all of which are eliminated. The coated wire is treated in the same way as the copper strandthe die D, or another of the same size, being placed at the back of the cylinder and a larger one substituted at the front. A second coating is then laid on, and after it passes through a similar process of examination a third coating is applied, and so on until the requisite number is completed. The finished core changes rapidly in its electric qualities at first, and is generally kept for a stated interval of time before being subjected to the specified tests. It is then placed in a tank of water and kept at a certain fixed temperature, usually 75 F., until it assumes approximately a constant electrical state. Its conductor and dielectric grip the drum P, round which it passes several times to prevent slipping. On the same shaft with P is fixed a brake-wheel furnished with a powerful brake B, by the proper manipulation of which the speed of paying out is regulated, the pull on the cable being at the same time observed by means of D. The shaft of P can be readily put in gear with a powerful engine
of Submarine Cable. drum, and thus rotating at the speed of the outgoing cable; on this screw works a nut which forms the centre of a thin circular disk, the edge of which is pressed against the surface of a right circular cone
nut and screw are rotating at the same speed, the position of the former will remain fixed; and as the nut is driven by friction from the surface of the cone, this equality of speed will obtain only when the product of the diameter (d) of the cone at that position multi-plied into its speed of rotation (n) equals the product of the diameter (a) of the disk multiplied into the speed of rotation (N) of the screw, or N/n=d/a, and thus the ratio of cable paid out to that of wire paid out is continuously given by a pointer controlled i See Sir W. Thomson (Lord Kelvin) Mathematical and Physical Papers, vol. ii. p. 165. by the disk, for any difference in speed between nut and screw will cause the nut to move along the screw until the diameter of the cone is reached which fulfils the above conditions for equality in speed. In fig. i i the edge of the disk serves as the pointer and the scale gives the percentage of slack, or (Nn)/n. The wire being paid out without slack measures the actual distance and speed over the ground, and the engineer in charge is relieved of all anxiety in estimating the depth from the scattered soundings of the preliminary survey, or in calculating the retarding strain required to produce the specified slack, since the brakesman merely has to follow the indications of the instrument and regulate the strain so as to keep the pointer at the figure requiredan easy task, seeing that the ratio of speed of wire and cable is not affected by the motion of the ship, whatever be the state of the sea, whereas the rl~li 'i/i/r/19/ i/ihUhphh/Ihhli/i o q io !o ip do FIG. ii.Slack Indicator strain will in heavy weather be varying 50 per cent. or more on each side of the mean value. Further, the preliminary survey over the proposed route, necessary for deciding the length and types of cable required, can afford merely an approximation to the depth in which the cable actually lies, since accidents of wind and weather, or lack of observations for determining the position, cause deviations, often of considerable importance, from the proposed route. From the continuous records of slack and strain combined with the weight of the cable it is a simple matter to calculate and plot the depths along the whole route of the cable as actually laid. Fig. 12, compiled from the actual records obtained during the laying of the Canso- Fayal
close adherence to the proposed route. The ordinates of the curve give the strain in cwts., and the abscissae the distance in miles measured from the Canso end; as the strain is proportional to the depth, 18 cwts. corresponding to i000 fathoms, the black line represents to an exaggerated scale the contour of the sea bed. Owing to the experience gained with many thousands of miles of cable in all depths and under varying conditions of weather and climate, the risk
The grappling of the cable and raising it to the surface from a depth of 2000 fathoms seldom occupy less than twenty-four hours, and since any extra strain due to the pitching of the vessel must be avoided, it is clear that the state of the sea and weather is the predominating factor in the time necessary for effecting the long series of operations which, in the most favourable circumstances, are required for a repair. In addition, the intervention of very heavy weather may mar all the work already accomplished, and require the whole series of operations to be undertaken de novo. As to cost, one transatlantic cable repair cost 75,000; the repair of the- Aden-Bombay cable, broken in a depth of 1900 fathoms, was effected with the expenditure of 176 miles of new cable, and after a lapse of 251 days, 103 being spent in actual work, which for the remainder of the time was interrupted by the monsoon; a repair of the Lisbon-Porthcurnow cable, broken in the Bay of Biscay in 2700 fathoms, eleven years after the cable was laid, took 215 days, with an expenditure of 300 miles of cable. All interruptions are not so costly, for in shallower waters, with favourable conditions of weather, a repair may be only a matter of a few hours, and it is in such waters that the majority of breaks occur, but still a large reserve fund must be laid aside for this purpose. As an ordinary instance, it has been stated that the cost of repairing the Direct United States cable up to 1900 from its submergence in 1874 averaged 800o per annum. Nearly all the cable companies possess their own steamers, of sufficient dimensions and specially equipped for making ordinary repairs; but for exceptional cases, where a considerable quantity of new cable may have to be inserted, it may be necessary to charter the services of one of the larger vessels owned by a cable-manufacturing company, at a certain sum per day, which may well reach 200 to 300. This fleet of cable ships now numbers over forty, ranging in size from vessels of 300 tons to 10,000 tons carrying capacity. The life of a cable is usually considered to continue until it is no longer capable of being lifted for repair, but in some cases the duration and frequency of interruptions as affecting Life. public convenience, with the loss of revenue and cost of repairs, must together decide the question of either making very extensive renewals or even abandoning the whole cable. The possibility of repair is affected by so many circumstances due to the environment of the cable, that not even an approximate term of years has yet been authoritatively fixed. It is a well-ascertained fact that the insulator, gutta-percha, is, when kept under water, practically imperishable, so that it is only the original strength of the sheathing wires and the deterioration allowable in them that have to be considered. Cables have frequently been picked up showing after many years of submergence no appreciable deterioration in this respect, while in other cases ends have been picked up which in the course of twelve years had been corroded to needle points, the result probably of metalliferous deposits in the locality. It is scarcely possible from the preliminary survey, with soundings several miles apart, to obtain more than a general idea as to the average depth along the route, while the nature of the constituents of the sea bed can only be revealed by a few small specimens brought up at isolated spots, though fortunately the globigerine ooze which covers the bottom at all the greater ocean depths forms an ideal bed for the cable. The experience gained in the earlier days of ocean telegraphy, from the failure and abandonment of nearly 5o per cent. of the deep-sea cables within the first twelve years, placed the probable life of a cable as low as fifteen years, but the weeding out of unserviceable types of construction, and the general improvement in materials, have by degrees extended that first estimate, until now the limit may be safely placed at not less than forty years. In depths beyond the reach of wave motion, and apart from suspension across a submarine gully, which will sooner or later result in a rupture of the cable, the most frequent cause of interruption is seismic or other shifting of the ocean bed, while in shallower waters and near the shore the dragging of anchors orfishing trawls has been mostly responsible. Since by international agreement the wilful damage of a cable has been constituted a criminal offence, and the cable companies have avoided crossing the fishing banks, or have adopted the wise policy of refunding the value of anchors lost on their cables, the number of such fractures has greatly diminished. Instruments for Land Telegraphy.At small country towns or villages, where the message traffic is light, the Wheatstone " A B C " instrument is used. In this apparatus electricA B C currents are generated by turning a handle (placed in instru- front of the instrument), which is geared, in the instru- ment. ments of the most recent
At offices where the work is heavier than can be dealt with by the A B C apparatus, the " Single Needle " instrument has been Single very largely employed; it has the advantage of slight needle liability to derangement, and of requiring very little instru- adjustment. A fairly skilled operator can signal with it ment. at the rate of 20 words per minute. The needle (in the modern pattern) is of soft iron, and is kept magnetized inductively by the action of two permanent steel magnets. The coils are wound with copper wire (covered with silk), to mils. in diameter, to a total resistance of 200 ohms. The actual current required to work the instrument is 3.3 milliamperes (equivalent approximately to the current given by t Daniell cell through 3300 ohms), but in peactice a current of to milliamperes is allowed. A simple, but important, addition to enable the reading from the instrument to be effected by sound is shown in fig. 13; in this arrangement the needle strikes against small tubes formed of tin-plate. Although a most serviceable instrument and cheap as regards maintenance, the " single needle " has (except for railway telegraph purposes) been discarded in favour of the " sounder," to secure the advantage of using one general pattern of apparatus, as far as possible, and to avoid the necessity of two different types of instrument being learnt by the telegraphist. The well-known code of signals (fig. 14) introduced by Morse is still employed in with sounding arrange- international code in vogue in Europe ment. differs only slightly from it. The instruments used for land telegraphs on this system are of two types" sounders," which indicate by sound, and " recorders," which record the signals. Recorders vary in details of construction, but all have the same object, namely, to record the intervals during which the current is applied to the line. In the earlier forms of instrument the record was made by embossing lines on a ribbon of paper by means of a sharp
almost universally used in Europe makes the record in Morse ink, and hence is sometimes called the "ink-writer." writer. This method has the advantage of distinctness, and so is less trying to the eyes of the operators. Although the " ink-writer " is still in use it is practically an obsolete instrument, and has been displaced by the " sounder." Operators who used the recorder soon learned to read the message by the click of the armature against its stop, and as this left the hands and eyes free to write, reading by sound was usuallypreferred. Thus, when it is not necessary to keep a copy, a much simpler instrument may be employed and the message read sounder, by sound. The earliest successful form was " Bright'sbell " sounder, which consisted, of two bells of distinct tone or pitch, one of which was sounded when the current was sent in one INTERNATIONAL CODE A O --- 4 B ... P -- 5 C - - Q -- 6 C h ---- 7 D g ... 8 E T 9 U 0 G-- V ... O 3 P 4 Q ... 8 R .. 6 S 7 T 8 U 9 0 J K Y .. .. , ..-- L Z t ---- M -- 1 .---. & ... N direction and the other when it was reversed. This instrument was capable of giving very considerable speed, but it was more complicated than that now in use, which consists only of an electromagnet, with its armature lever arranged to stop against an anvil or screw in such a way as to give a distinct and somewhat loud sound. Dots and dashes are distinguished by the interval between the sounds of the instrument in precisely the same way as they are distinguished when reading from the recorder by sound. Fig. 15 shows the modern pattern of " sounder " as used by the British Post Office. The magnet is wound to a resistance of 40 ohms (or 900 ohms when worked from accumulators), and the instrument is worked with a current of 400 milliamperes (25 milli-amperes with accumulators). Methods of Working Land Circuits.The arrangement on the " open-circuit " system for single-current working is shown in fig. 16, in which Li represents the line, G a galvanometer, used simply to show that the currents are going to line when the message is being transmitted, K the transmitting key, B the battery, I the receiving instrument, and E the earth-plate. The complete circuit is from the plate E through the instrument I, the key K, and the galvanoscope G to the line Li, then through the corresponding instruments to the earth-plate E at the other end, and back through the earth to the plate E. The earth is always, except for some special reason, used as a return, because it offers little resistance and saves the expense and the risk of failure of the return wire. The earth-plate E ought to be buried in moist earth or in water. In towns the water and gas pipe systems form excellent earth End of Article: IIII If you wish, you can link directly to this article.
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