relays should be cut off from the main line, but in such a manner as not to break it.

572. The Circuit. In what has been said, only a single wire has been spoken of as running from station to station. This is generally an iron wire which passes over glass insulators attached to tall wooden posts. When the wires are laid under ground or in water, they are insulated by a coating of gutta-percha. Copper wires are commonly used in offices.

It would seem necessary, in order to complete the circuit, that a second wire should be employed. Such, however, is not the case. The employment of a second wire is avoided by connecting the two ends of the single wire with the earth. When gas or water pipes enter an office the ground wire is attached to them.

If there are no such conveniences, copper plates several feet square at each station are buried in a perpendicular position, at sufficient depth so as to be always in contact with moist earth. The circuit is thus completed. At the station where the message is sent the line is connected with the positive pole of the battery, and the current passes over the wire down through the earth back to the negative pole. This simple device saves not only half the expense in constructing wires, but greatly increases the power of electrical transmission, the resistance it offers compared with the wire being practically nothing.

Since the earth is the common reservoir of neutral electricity the electric current from the wire is really dissipated when it communicates with it. There is not supposed to be any real passage of the electricity back to the battery from which it started. The intermediate offices are supplied with ground wires, to be used only in case of trouble on the line.

573. Plan of a Way-Station. In Fig. 392 we have represented a plan of the instruments and connections of a way-station. The line enters at L, passes through the lightning arrester, X, traverses the coil of the relay, M, and then passes through the key, K, back to the lightning arrester, and then to the next station by the line, L'. The dotted lines

represent the local circuit, and E the local battery that is operated by the relay, M. The sounder, S, which is included in the circuit, is also operated. Instead of the sounder a register might be substituted.

By turning the button, C, the current passes along the main line without going through the instrument. The switch, Q, can be used

to connect the line on either side of the instruments with the earth at G. The line connected with the ground at G can then be worked independently of the other.

A plan of a terminal station would have a large main battery, and would dispense with the "cut-out," C, and the line, L'; in other respects it would agree with the plan of a way-station.

574. Other Forms of Telegraphs. Other systems of telegraphy besides MORSE'S are used to a greater or less extent. The most ingenious of these are BAIN's electro-chemical telegraph, and the printing telegraphs of HOUSE, HUGHES, and PHELPS.

In BAIN'S telegraph there is no magnet used, but a small steel point connecting with the line wire, when the current passes, presses upon the roll of paper that has been previously soaked in ferrocya

nide of potassium, and which rests upon a metallic plate. When the point touches the paper the chemical preparation is decomposed, and blue marks are left on the paper, due to the formation of Prussian blue.

Of the three printing telegraphs, that of PHELPS is the most serviceable, and is a combination of HOUSE'S and HUGHES's with the improvements of PHELPS.

The sending instrument has twenty-eight keys arranged like those of a piano; upon these are printed the twenty-six letters of the alphabet, and two punctuation points. When the operator depresses the keys, the circuit is closed, and the message is printed at the other end of the line in ordinary letters. This system works faster than MORSE'S, and the message does not have to be transcribed.

575. Duplex and Quadruplex Telegraphy. - Duplex telegraphy refers to that system of telegraphing by which messages are simultaneously sent in opposite directions on one and the same wire, thereby doubling the working capacity of the line. Quadruplex telegraphy refers to the system of telegraphing whereby four messages, two in each direction, may be simultaneously transmitted over one and the same wire. The quadruplex systein has been extensively employed within a few years by the Western Union Telegraph Company, and is at present in use between almost all the principal cities in the country.

the

A detailed description of these systems, however, would be beyond scope of the present work.

Not

576. Submarine Cables. Since the invention of the telegraph, a complete network of lines has been established over both continents. only have thousands of miles of wires been stretched on land, but submarine wires have been laid, connecting places separated by thousands of miles of water. Telegraphic wires connect England and Ireland, England and France, France and Algiers, Europe and America.

Fig. 393.

The Atlantic cables (Fig. 393) consist of (1) a central conducting strand, O, of seven copper wires; (2)

surrounding this an insulating coat, C, of gutta-percha; (3) a layer of several strands of tarred hemp, H, to protect the gutta-percha; and (4) outside of all, eighteen iron wires, I, as a protecting sheath.

The signals are indicated by means of Sir WM. THOMSON'S reflecting galvanometer, an instrument of extreme delicacy, without which the practical success of the Atlantic cable would have been a matter of great doubt.

A needle with a very light mirror attached is suspended by a silk thread within a coil of insulated copper wire; at a distance of a yard is a scale with zero in the centre and the graduation extending on each side. Under the zero point is an aperture through which the light comes from a gas or lamp flame, and strikes on the mirror and is then reflected upon the scale. The spot of light is deflected to the right or left according to the current, and these deflections correspond to the dots and dashes of MORSE's alphabet.

577. The Fire-Alarm Telegraph. - Electricity is now widely used in many places to indicate the locality of fires. The apparatus employed is really a modification of MORSE'S telegraph. In various parts of the city or town are small boxes, called signal-boxes, which are connected with a central station by means of wires.

When a fire occurs in the vicinity of any box, by turning a crank within the box the circuit is opened and closed, and the number of the box thereby telegraphed to the central station. This station is also connected with bells by wires at different points, and when the watchman on duty here receives notice of a fire as stated above, he strikes, by means of the electric current, on the bells the number of the box in whose neighborhood the fire is, so that the firemen know almost the precise locality of the fire.

578. Electro-magnetic Motor. Many attempts have been made, and with partial success, to employ electromagnetism as a motor for the propulsion of machinery, but in all cases the expense has been so great as to preclude its economical use.

Fig. 394 represents an electro-magnetic machine. It is composed