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Also when using cables from a power supply to power a project or circuit being developed, it can be very convenient and very much tidier to have the power cables twisted together to prevent them being tangled an getting in the way of more important items. In both instruments the signals are sent by means of a perforated ribbon of paper; but the cable sender was the more complicated, because the cable signals are formed by both positive and negative currents, and not merely by a single current, whether positive or negative. The speed of signalling by the siphon recorder is of course regulated by the length of cable through which it is worked. The principle of the siphon recorder is exactly the inverse of the mirror galvanometer. We owe it to his admirable inventions, the mirror instrument of 1857 and the siphon recorder of 1869, that messages through long cables are so cheap and fast, and, as a consequence, that ocean telegraphy is now so common. Sir William (then Professor) Thomson first solved the difficulty by his invention of the 'mirror galvanometer,' and rendered at the same time the first Atlantic cable company a commercial success.

The moving body must be quite free to follow the undulations of the current, and at the same time must record its motions by some indelible mark. At 9 a.m. a message from England cited these words from a leading article in the current Times: 'It is a great work, a glory to our age and nation, and the men who have achieved it deserve to be honoured among the benefactors of their race.' 'Treaty of peace signed between Prussia and Austria.' The shore end was landed during the day by the Medway; and Captain Anderson, with the officers of the telegraph fleet, went in a body to the church to return thanks for the success of the expedition. Captain (now Sir) James Anderson, of the Cunard steamer China, a thorough seaman, was appointed to the command, with Captain Moriarty, R.N., as chief navigating officer. It was to supply such a desideratum for cable work that Sir William Thomson invented the siphon recorder, his second important contribution to the province of practical telegraphy. The 'duplex' system, or method of telegraphing in opposite directions at once through the same wire, has of late years been applied, in connection with the recorder, to all the long cables of that most enterprising of telegraph companies-the Eastern-so that both stations may 'speak' to each other simultaneously.

The recorder, which was shown in operation, naturally stood in the place of honour, and attracted great attention. About the same time he revived the Sumner method of finding a ship's place at sea, what are electric cables and calculated a set of tables for its ready application. Now the tendency of a charged body is to move from a place of higher to a place of lower potential, and consequently the ink tends to flow downwards to the writing-tablet. At the same time, the waves of electromagnetic field move rapidly forward. The electrician knew that the mistral would blow before long, and, as it rarely blows for less than three days at a time, that rather rude wind, so dreaded by the Marseillaise, was doubly dreaded by him. On short cables, say under three hundred miles long, they are rarely, if ever, used. With fibre-optic cables, made of flexible fibres of glass and plastic, electrical signals are converted to light pulses for the transmission of audio, video, and computer data. Babbage was among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light; but Sir William pointed out the merits of the Morse telegraphic code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes.

To introduce his apparatus for signalling on long submarine cables, Sir William Thomson entered into a partnership with Mr. C. F. Varley, who first applied condensers to sharpen the signals, and Professor Fleeming Jenkin, of Edinburgh University. Mr. (afterwards Sir) Samuel Canning was engineer for the contractors, the Telegraph Construction and Maintenance Company, and Mr. de Sauty their electrician; Professor Thomson and Mr. Cromwell Fleetwood Varley were the electricians for the Atlantic Telegraph Company. Construction cranes are not waterbirds, in the same way that flowing energy (watts) is not flowing charge (amperes.) If a flow of electricity is an electric current, then electricity cannot be a form of energy. The construction of long submarine cables for either telephone or telegraph service is somewhat different from that discussed previously. These cables are composed of materials that are electrically conducting with an outer insulating cover. The materials each have different resistances. In the former we have a small coil suspended between the poles of a large magnet-the magnet enclosing the coil, which is also free to rotate round its own axis. At Suez, where they suffer from drought, a cloud of steam was kept rising round the instrument, saturating the air and paper.

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