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tricity by means of little pulsating drums immersed in a vessel of water; when two drums pulsate together in time they repel each other, even as two similarly electrified particles of matter repel each other; and when they pulsate out of time they attract each other, just as two dissimilarly electrified particles attract each other. May it not be then that the atoms of matter steeped in the ether are like these pulsating drums in the liquid water, and attract or repel each other according as their vibrations are harmonious or discordant ?

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THE first electric telegraph put to any practical use was a short line erected in 1833 by Gauss and Weber, the celebrated German physicists, to connect the Observatory with their physical cabinet at the University of Gottingen. In 1837 Messrs. Cooke and Wheatstone applied their needle telegraph instrument to a

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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:
1. The apparatus for sending the message.
2. The line for conveying it.

3. The apparatus for receiving the message.

L

m

E'

FIG. 16.

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 m1, 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 crossarm 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 b 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 lightning currents in the wire endeavouring to leap to earth through the substance

[graphic]

FIG. 17.

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

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