It is the usual practice for the manufacturer to lay the cable and test it to ascertain whether the specified elec trical characteristics have been met before turning it over to the operating company. The ships used for laying are specially con structed to carry cable in cylindrical tanks which are built into the ship's structure. The largest ship of its kind anywhere in the world, the C.S. "Dominia," which is both owned and oper ated by the Telegraph Construction and Maintenance Company has four of these tanks, which have a combined net coiling capacity of i9o,000 cubic feet. In them may be stowed away a cable over 3,00o m. long. The cable is coiled down around a cen tral hub-shaped cone in horizontal layers, starting at the tank wall and coiling in toward the centre. When the cone is reached the cable is run out radially to the wall, again with wedge-shaped strips of wood, called "feather-edges," on either side to protect it from the weight of the layers above, and the process is repeated. When paying out the cable it is pulled through a circular hatch in the deck above the cone, several hands being stationed in the tank to guard against kinks forming, or the cable fouling. In load ing the vessel the cable from the top of one tank is led over into the bottom of the adjacent tank, making it necessary only to slacken the speed of paying out when changing over from one tank to the next.
From the tank the cable passes to the paying out gear. This machinery for controlling the stress on the cable is placed both at the bow and stern. All long pay-outs are made over the stern, but it is of ten more convenient to pay out short sections over the bow, as it is to this point that the cable is brought for splicing. The machine consists of a large drum 6 ft. in diameter, around which several turns of cable are taken to provide the necessary friction. On the same shaft is a series of water-cooled adjust able band brakes and connected to it is a large fan brake. The cable then passes through a dynamometer, which gives an indica tion of the stress, leaving the ship over the bow or stern guide, as the case may be. It is not uncommon for the stress on the cable in paying out in deep water to amount to as much as two tons. In order to check the amount of slack in the cable there is paid out at the same time a taut steel piano wire which is carefully measured and compared with the length of cable paid out. When passing over fairly smooth bottom the amount of slack paid out rarely exceeds o%, but care is taken not to leave the cable in a state of tension on an uneven bottom, or suspended in bights over rocks, etc., as injury to the cable would result.
The heavy shore end of the cable is landed from a smaller ship which is moored close to the beach and a line sent ashore, sup ported by barrel floats. When enough of the cable has been landed the barrels are cut away. The remaining length on the smaller ship is then paid out and the end buoyed in water sufficiently deep to permit the deep sea ship to approach. The latter takes the
buoyed end aboard over the bow, a splice is made, and the cable transferred to the stern. The ship then starts on her long voyage. When approaching her destination the cable is cut and the end buoyed in deep water. When this shore end of the cable has been laid out to meet the buoyed deep-sea end, both ends are taken aboard and tested; the final splice is then made and the cable dropped over the bow. During cable laying testing is carried on continuously by a staff of electricians, whose duty it is to detect and report any fault in the cable.
Should the cable develop a fault or break while being laid, the ship is equipped with special means of coping with the trouble. From the results of electrical testing it is possible to arrive at the length of cable between the ship and the fault.
When trouble develops in a submarine cable it is usually possible to localize the fault or break with a fair degree of accuracy by determining the electrical resistance or capacity of the core between the testing point and the point of failure by the Wheatstone bridge method. Knowing the char acteristics of the cable it is possible to deduce the distance in miles, which interpreted in conjunction with the charted route enables the navigator of the repair ship to place a mark buoy within work ing range. In deep water it is necessary to use a cutting grapnel which automatically grips the cable and cuts away one end when it is lifted. The very ingenious construction of the Lucas grapnel permits it to be set so as to cut away whichever end is desired. A suddenly increased stress on the grapnel rope which passes through the dynamometer gives an indication when the prongs of the grapnel encounter an obstacle. The vessel is stopped and the picking-up gear is started. When the cable has been raised to the surface it is secured with a rope, or chain stopper and taken aboard. Test leads are attached to determine the electrical condition of the cable and the fault is localized. When the line is electrically clear to one of the shore stations the end is buoyed. Should the cable be in danger of parting, a length of new cable from the ship's tanks is spliced on and paid out until all the cable suspended off the bottom is new. The ship is then free to go after the cable on the other side of the fault. When this end has been raised and the tests indicate its continuity and satisfactory insula tion all the way to the shore station, cable from the ship's tanks is spliced on and paying out commences towards the buoyed end, which, when it is reached, is taken aboard. Final tests are taken in both directions; the cable from the ship's tanks is stoppered off and cut, and the final splice is made on the ship's deck in a bight both sides of which are secured with stoppers, by means of which it is eased over the bow and the lines holding it are cut away.
The simplex method—sending single messages in one direction only—of operating submarine cables is not generally