viable. Some have been used as scientific instruments to measure earthquake waves and other geomagnetic events. Other uses In 1942, Siemens Brothers of New Charlton, London, in conjunction with the United Kingdom National Physical Laboratory, adapted submarine communications cable technology to create the world's first submarine oil pipeline in Operation Pluto during World War II. Active fiber optic cables may be useful in detecting seismic events which alter cable polarization. Modern history Optical telecommunications cables In the 1980s, fibre-optic cables were developed. The first transatlantic telephone cable to use optical fibre was TAT-8, which went into operation in 1988. A fibre-optic cable comprises multiple pairs of fibres. Each pair has one fibre in each direction. TAT-8 had two operational pairs and one backup pair. Except for very short lines, fibre-optic submarine cables include repeaters at regular intervals. Modern optical fibre repeaters use a solid-state optical amplifier, usually an erbium-doped fibre amplifier. Each repeater contains separate equipment for each fibre. These comprise signal reforming, error measurement and controls. A solid-state laser dispatches the signal into the next length of fibre. The solid-state laser excites a short length of doped fibre that itself acts as a laser amplifier. As the light passes through the fibre, it is amplified. This system also permits wavelength-division multiplexing, which dramatically increases the capacity of the fibre. Repeaters are powered by a constant direct current passed down the conductor near the centre of the cable, so all repeaters in a cable are in series. Power feed equipment is installed at the terminal stations. Typically both ends share the current generation with one end providing a positive voltage and the other a negative voltage. A virtual earth point exists roughly halfway along the cable under normal operation. The amplifiers or repeaters derive their power from the potential difference across them. The voltage passed down the cable is often anywhere from 3000 to 15,000VDC at a current of up to 1,100mA, with the current increasing with decreasing voltage; the current at 10,000VDC is up to 1,650mA. Hence the total amount of power sent into the cable is often up to 16.5 kW. The optic fibre used in undersea cables is chosen for its exceptional clarity, permitting runs of more than between repeaters to minimize the number of amplifiers and the distortion they cause. Unrepeated cables are cheaper than repeated cables and their maximum transmission distance is limited, although this