Page:The New International Encyclopædia 1st ed. v. 19.djvu/119

TELEGRAPH.

Owing to the fact that in some of the early telegraphs used in Europe the space could not be employed as part of any of the signals without causing confusion, it was necessary to rearrange the combinations of dots and dashes, so as to form the whole alphabet with the dot and dash without the use of the space. This alphabet is known as the Continental Morse alphabet. It is used throughout Europe and in all submarine telegraphy, having been adopted by an international conference.



For the generation of power in the electric telegraph, the gravity batteries (see or ) are chiefly employed in the United States. Various forms of the Daniell battery are also used, especially in Europe. The number of cells employed varies with the length of the line, the conditions of the wire as regards insulation, and the nature of the instruments used. It is quite common in the United States, especially in cities where many lines centre, to employ dynamos driven by steam engines for generating the current in place of batteries, or to make use of storage batteries.

The mode of joining up two or more stations by means of the line wire is shown in the figure. It being necessary to furnish a complete circuit or path for the current from the battery over the line and back to the battery, a return connection is required. In practice, this return connection is made by simply connecting the instruments at each station with the earth.



Assuming S and S' to be the telegraph stations, with the wires grounded at P and P' (see subsequent paragraph), B and B' are the batteries, M and M' the sounders, while K and K' are the

transmitting keys. The connection at the key is always completed by a supplementary switch, when the key is not being used to send a message, in order that the path may be complete for the reception of any message from the other office.

In sending a message the switch is first opened and then the key is depressed. A current of electricity will pass from the battery along the line wire, around the magnets, drawing down the armature and making a click; thence down to the earth plates and through the earth back to the battery.

The operator at the sending station in transmitting a message opens his switch and depresses his key, so as to send a series of dots and dashes corresponding to the letters to be sent to the distant station. These are heard by the receiving operator, who writes down their meaning as fast as received. The above represents the closed-circuit system, such as is employed in the United States and Canada. In the open-circuit system the current flows over the wires and through the instruments only when the sending operator depresses his key. This involves a battery at each station, which is used, however, only when signals are being transmitted.

The wire or series of wires are usually supported on poles placed along the sides of a road or railway. In cities the wires are sometimes carried over houses or in underground pipes or conduits, the conductors in the latter case being insulated by means of a gutta-percha or other suitable covering. In pole and over-house lines, the wires are kept from each other and the current from escaping to the earth by insulators of glass. In Europe white porcelain and brown stoneware insulators are used, and the former substance, when of good quality, well glazed and well burned, is perhaps the most perfect of all insulating materials, and does not deteriorate with age. The fewer the poles on which the wires are suspended, the better is the insulation and the less the cost, but the liability to accident is greater. The number of poles used varies from 30 to 50 per mile, and is governed by the number of wires carried, the configuration of the road or track, and other considerations. On road lines, the number of poles is generally larger than in the case of wires carried alongside railways, the greater level and straightness of the latter reducing the number of supports required, and thirty-five to forty to the mile is considered an average. The wire chiefly used for overland telegraph purposes is of iron, galvanized, and of No. 8 (1-8 inch) or No. 6 (1-6 inch) gauge, or copper of somewhat smaller diameter, the latter being preferred on account of its greater conductivity. The conductivity of a wire increases in the ratio of the square of its diameter (the resistance decreasing in inverse ratio), and the