TELEGRAPHY, SUBMARINE. In submarine telegraphy there are many variations from the practice on land lines. (See TELEGRAPH.) This is the ease because the length of the line between stations is usually great and certain conditions attend the construction and laying of the cable bearing the conductor. The problem then in volves the transmission of current along a con ductor of great length and necessarily small cross-section with of course considerable resist ance. In a long cable we have to consider the effect of the electrostatic inductive capacity as well as its conductivity. The cable acts as a con denser (q.v.), the core o• conductor forming one of the plates or conducting surfaces, while the metallic sheathing acts as the other. As it takes a perceptible time for the cable to he charged and discharged when a current is sent through it, there is a certain limit to the speed of transmission of signals without their becoming confused and unintel ligible. As the length of the cable increases, so does the time required for charging, and it was demonstrated by Lord Kelvin (1853) that the rate of signaling would vary inversely as the square of the length of the cable. So small is the current passing over the cable that it is impossible to use ordinary telegraph instruments on lines greater than 500 miles in length, and only up to 150 miles can an ordinary rate of signaling be maintained. It is necessary, therefore, to lie some sensitive device such as a mirror galvanometer or a Thomson siphon re corder to receive the transmitted signals. The galvanometer (q.v.) or some modified o• special form of this instrument when used in cable teleg raphy is of high resistance and considerable sen sitiveness. '1'110 ordinary mirror receiver re sembles the Thomson reflecting galvanometer, and has one coil and a suspended mirror to which the magnets are attached. The action is precisely the same as in the galvanometer, the amount and direction of the deflection depending on the in tensity and direction of the current. A gal vanometer with a suspended coil may also be used, or, what is more usual, some form of siphon recording receiver. The siphon recorder was in vented by Lord Kelvin in 1867 and has since been greatly modified and improved by various cable engineers. It consists essentially of a coil of light wire suspended between the poles of a powerful magnet, capable of being deflected from its position of rest upon the passage of a cur rent. As the coil moves its motion is transmitted to a siphon formed by a fine glass tube, one end of which dips into a vessel containing ink. while the other is near or in contact with a strip of paper which is moved through the apparatus by a motor, either electrical or clockwork. This glass pen will trace on the tape a line where the move ments of the coil are indicated by curves o• waves marked on the paper. These curves are above
or below' the line of rest, depending upon the di rection of the current which passes through the coils. Thus to produce a dot a positive current is sent over the wire and an upward curve is pro duced on the record. Reversing the current gives a dash, and employing the Continental Morse al phabet described under TELEGRAPHY, the various signals can be transmitted. With such recorders are employed either simple keys or automatic transmitters. The simple key consists of a pole changing device by which a current in either di rection can be sent over the circuit by simply pressing one of two keys. In automatic trans mission, which is used on the transatlantic cables and other busy lines, the operator first punches in a paper strip holes corresponding to the proper signals, and then contact is made through the agency of rods or brushes as the tape is passed through the transmitter. These devices, however, can only be used on cables having heavy cores, but by their use a rate of 50 words or more a minute may be obtained.
In working cables it was soon found that their efficiency was increased and the effect of earth currents eliminated by inserting a condenser (q.v.) between the transmitting instrument and the line. This idea, originated by C. F. Valley, was embodied in a patent in 1862 and put in practice in 1866. When first used at the receiv ing end it was found that the speed was consider ably increased, and when placed at both ends the efficiency of the cable was practically doubled. Greater distinctness in the transmission of the signals was obtained as well as increased rapid ity. The operation of a simple cable circuit will be seen from the preceding diagram of the con nections: The cable connected with a condenser at either end is shown above. From the opposite plates of the condenser connection is made with a switch through which the current may pass either to the earth through a galvanometer or recorder or to a transmitting key, K, and then to the battery and earth, depending on whether signals are be ing sent or received at the particular station. At K and K' either pole of the battery can be con nected at will to the line, or rather to the con denser, and the cable charged inductively. The corresponding signals are produced at the distant station, the galvanometer being connected to the line by the switch. Cables can also be worked du plex with facility and an increased speed of 90 per cent. attained, but as yet it is not possible to employ the quadruplex method in use on land lines. The duplex system was devised in 1S75 and 1876 by Muirhead and Taylor and was first used on the Atlantic cable in 1S78. Previously in 1373 experiments in this field had been car ried on by J. B. Stearns.