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wire laid along the Great Western Railway, and in 1839 this line was extended as far as West Drayton, a distance of thirteen miles. The clever capture by its aid of a Quaker named Tawell, for a crime committed at Slough, brought the new invention into public note, and gave an important impulse to the development of the telegraph in England. This was the first telegraph line conveying public messages, and though Samuel Morse is fondly called the “Father of the Telegraph by Americans, he is only the father of the American telegraph. Rightly viewed, the telegraph is not the work of any single man, and though the Morse telegraph instrument was invented as early as 1835, and publicly tried in 1837, the first telegraph line was not erected in the United States till 1844.
A land telegraph circuit consists of three parts :
Such a circuit is shown in Fig. 16, where b, m, and k is the sending apparatus, and b', m', k the receiving apparatus, connected together in one complete circuit by the line wire L and the "earth-plates " E' E, with the ground between. For though the electricity must, as we have already seen, have a complete course to flow in from one pole of the battery back to the other, it need not be entirely made up of wire. A wire is necessary to convey the current from one pole of the sending battery b to the distant receiver mủ, but it can return to the other pole through the earth itself if it be properly led into the ground. This is done by means of copper sheets buried in the ground at each station, and connected by wires to the apparatus and the line.
The telegraph line consists of the wire conveying the current, the poles supporting the wire above the ground, and the insulators which isolate the wire from the poles. The wire is usually of iron (No. 8 Birmingham wire gauge), protected from rusting by galvanising, or, in other words, coated with a thin layer of zinc. Wires of phosphor-bronze or steel cased in copper have also been introduced for overhead wires, but not to any great extent as yet. The poles are generally of larch wood in this country, but iron poles are frequently sent abroad to South Africa and other places where timber is rare or the white ant is too fond of it. The insulator is simply a prop of nonconducting substance, such as glass or earthenware, inserted between the wire and pole to keep the current from leaving the wire and flowing through the pole into the ground, and thus taking a short cut back to the battery. The material of the insulator should therefore be highly insulating, and its shape should be such that rain or dews collected on its surface should not conduct the electricity to the pole. One of the best insulators is that of Mr. C. F. Varley, illustrated
in section, Fig. 17. It consists of two inverted porcelain cups a, b, placed one over the other. The inner cup screws into the outer and is cemented to an iron stem c, which is supported from a wooden cross
arm d carried by the pole. The wire e is bound to a groove in the side of the insulator by finer binding wire, and the electricity can only escape to the pole by traversing the whole
surface of the outer and % inner cups. As the inner
cup is well sheltered from wet, it is very rarely that its surface becomes damp,
and hence the insulation d of the line keeps good.
To guard the line from damage by powerful light
ning currents in the wire FIG. 17.
endeavouring to leap to
earth through thesubstance of the insulator, each pole is fitted with a lightning-rod in the shape of an iron wire which runs up from the ground and taps the air above the pole.
A novel insulator has recently been introduced by Messrs. Johnson and Phillips. In it the porcelain. bell (b fig. 17) is curled inwards at the foot so as to form a circular channel all round the bottom. This is filled with a highly insulating mineral oil, on the surface of which dew cannot of course settle in a conductive film, and hence the insulation never falls below a certain value determined by the belt of liquid surface interposed between the wire e and the stem c. The shape of the new insulator is different from that of Mr. Varley's, but its principle will be understood from the figure to which we have referred.
The arrangement of the apparatus for sending and receiving a message is shown in Fig. 16. At each station a key, or sending instrument k, a battery b for supplying the current, and a receiving instrument m for signalling the message, is connected between the end of the line wire L and the “earth ”-plate E. The key is a simple device for opening or closing the circuit of the current, and stopping it or allowing it to flow at the will of the operator. In its simplest form it consists, as will be seen, of a short metal lever pivoted at the middle, like a sway-bar, and thus supported over the wooden base below. At each end of the lever there is a small contact stud of platinum, which is placed directly over a corresponding stud on the base ; so that when the operator grasps the knob or handle of the lever and works it up and down, the key makes and breaks contact with the studs below.
Now it will be seen from Fig. 16 that since the line wire is connected to the middle of the lever, and the bottom studs are respectively connected to the battery b and the receiving instrument m, the line wire can be put in connection either with the battery or the receiver by working the key, and the apparatus at either station can be put in the attitude of sending or receiving. Thus in the figure the position of the key on the right is that for sending a signal, and the position of the key on the left is that for receiving it. With such an arrangement only one station can send at a time, and the other must be in the attitude of receiving. There are arrangements of the apparatus, however, which enable both stations to send at the same time—and not only one message each, but even two or more. Such are the duplex, quadruplex, and multiplex systems of telegraphy.
The sending instrument which we have described simply makes and breaks the current from one pole of the battery, and the sense of the signals depends on the length of time the current is closed for each signal. Thus a short, or momentary closure, forms one elementary signal (technically called a “dot”), and a closure about three times as long forms the other elementary signal (known as a “dash"). An intelligent combination of these signals forms the message. Every letter of the alphabet has its equivalent in “ dots” and “dashes,” according to a code invented by Samuel Morse, and now universally employed in telegraphy. The following table gives the Morse Code, the short lines representing dots, and the long lines dashes: