On the first Atlantic cables the speed was about seven words a minute in one direction only. The speed of recent Atlantic cables is as high as from 40 to 45 words a minute in both directions — that is, from 80 to 90 words a min ute. Thus, compared with the early days, the speed and therefore the value of the best cables has been multiplied more than 10 times over by means of some of the most ingenious and deli cate machinery in modern industry. On the first Atlantic cable it was found that, using land telegraph methods of signaling, the speed was only one or two words a minute. The first great forward step was to send exceedingly fee ble currents and to use extremely sensitive receiving instruments. Lord Kelvin's mirror galvonometer supplied the instrument needed. By this means the speed of the early Atlantic cables was raised to seven or eight words a minute. Subsequently, when heavier cables were laid, the speed was increased to as much as 20 words a minute.
The Siphon Recorder.— In 1870 Lord Kel vin perfected his siphon recorder for working long cables, and it at once supplanted the mirror instrument, as it worked just as well with fee ble currents., gave a written record of signals received and enabled one man to do the work of two. An exceedingly light coil of fine wire (in shape and size like the long, narrow 0 which would be obtained by winding several hundred turns of fine silk thread around the palm of the open hand) is delicately suspended between the two poles of a powerful magnet. As the electric signals from the cable flow through the coil of wire, it swings round under the influence of the magnet, back or forward according as the current is positive or negative. The motions of the coil are transmitted by silken fibres to a little glass siphon about as thick as a needle and three or four inches long, suspended so as to swing with perfect freedom. One end of the siphon dips into a pot of ink, and the other end hangs close above a moving strip of paper. The signals are so feeble that if the end of the glass siphon rested on the paper it would not move at all, but by causing the siphon to vibrate continuously against the paper the free motion of the siphon is not inter fered with, and the ink is spluttered upon the paper so that the siphon traces a line of very fine dots and thus records the signals transmit ted through the cable. This instrument, though crude at first, has gradually been perfected. It is now the most important part of modern cable apparatus.
The Duple: System.— The next improve ment, undoubtedly the greatest ever made for increasing speed, was the•nvention of a success ful system of oduplexine cables by Dr. Alex
ander Muirhead and Herbert Taylor in 1875. This invention rendered it possible simultane ously to send messages both ways through a long ocean cable. In 1878 the Direct United States cable across the Atlantic was successfully duplexed, and a speed of 16 words a minute ob tained each way at the same time. Duplexing cables has now become such a fine art, chiefly through the labors of Dr. Muirhead, that the capacity of cables, and therefore their commer cial value, has been practically doubled. Since 1875 about 100,000 miles of ocean cables have been duplexed almost entirely on the Muirhead system.
The increasing traffic across the Atlantic and the pressure of competition led next to an in crease in the size of the copper °core' which conducts the electric signals. The resistance of a wire delays the electric current and therefore the speed. By doubling the size of the copper core the resistance is halved and the speed greatly increased. The copper wires used for telegraphy on land weigh about 200 pounds per mile. In 1894 two cables were laid across the Atlantic, one for the Commercial Cable Com pany and the other for the Anglo-American Company. The copper core of the former weighed 500 pounds per mile, while the latter weighed no less than 650 pounds per mile, or as much as three ordinary land wires. The result was that the speed obtained with these two cables was as high as 40 to 50 words a minute, or, working duplex, from 80 to 90 words a min ute. On previous Atlantic cables 25 to 28 words a minute was the maximum each way. Owing to the reduction of rates the benefit of this ten fold increase of speed since the early days has gone almost entirely to the general public.
Automatic Transmission.— The increase in speed brought up another difficulty. No human operator can send so fast. The key used for signaling through the cables by hand is practically the same as the ordinary Morse key used for land telegraphy, except that two keys are used side by side, one to send positive signals and the other negative signals, the let ters of the alphabet being indicated by various arrangements of the two kinds of signals. First-class cable operators can send as many as 30 words a minute for a few minutes, but a sustained speed of 20 words a minute, when working by the hour, is regarded as very good. To take full advantage of the speed of a mod ern Atlantic cable, therefore, it is necessary to have some automatic method of transmitting. The advantages of automatic transmission are higher speed, greater uniformity of signals, more legibility and fewer mistakes.