Gutta-percha and Sharks Teeth - hazards of the deep

This chapter, which I might have called 'technicalities without tears' provides a background to the events described previously. The earliest electrical telegraphs were on land, and it was comparatively easy to string naked iron or copper wire from pole to pole, for air is a good insulator and the glass or ceramic 'pots' prevented electrical leakage of current down the moist wood of the poles. To insulate a wire in the sea was another matter. It was not until the happy discovery that "gutta-percha", resin from a South AmericaN tree, was a good insulator that actually improved with time when immersed in salt water, that long distance submarine telegraphy became practicable.

Sharks teeth have been found embedded in retrieved cable, for fish it seems, are inquisitive. Teredo worms find gutta-percha sufficiently palatable to allow them to bore into it with as much pleasure as they attack ships timbers, and rough shallow seas and rocky bottomS pose their own hazards, as the early cable pioneers were soon to learn. The cable must be protected. Its insulated conductor is wrapped in layers of brass tape, tarred hemp, and steel sheathing wires until it looks more like a ships hawser than a path for tiny electrical currents. The early telegraph cables contained only a single conducting wire, for the sea (ground) was used as a return path - why lay two wires when it is possible to persuade the current to travel at least one way for free! Modern coaxial cables carrying voices as well as telegraph signals are more like pipes within pipes, but the basic techniques of laying and recovery in the deep oceans have links with those of the earliest days.

Ships anchors, trawlers 'otter-boards', even submarine earthquakes, have all been responsible for cable failure. Shore terminals can locate faults fairly precisely, the problem is often in actually picking up a damaged cable from the middle of the ocean. The cableship will often steam backwards and forwards across the charted line of the cable dragging a grapnel along the bottom. If the grapnel 'hooks' the cable this is shown by an increasing strain on the tow rope. Sophisticated instruments can measure this, but the most delicate response is sometimes obtained from the 'bosun's bottom'. He sits on a small board astride the taut tow rope and feels every vibration and jerk of the grapnel on its way across the rocky seabed, or the gentler taughtening caused by 'hooking' the cable. The actual repair may involve cutting out a mile or two of damaged cable and replacing it, with at least two splices required. This takes time, and the Captain hovers, his eye darting between the busy technicians and the approaching weather, for rough seas which do not endanger the ship can still make a repair operation impracticable.

The modern coaxial broadband cable carries a whole spectrum of telegraph and telephone calls simultaneously, rather as if the whole of the transmissions on the long, medium, and short wave bands on a radio had been thrust down a pipe instead of spread abroad on the 'ether' for all and sundry to hear. Amplifiers, fed with power from the shore terminals, boost this complex array of signals every few miles, and needless to say the same cable carries traffic in both directions at once. We have come a long way from the signal fires of Carn Brae.

Home | Site Map