net, which makes part of the receiver. It then enters the wire, T, which leads it to earth.

473. Morse's key and receiving instrument. The general arrangement of the apparatus being understood, the following are the details of its action. The key consists of a small mahogany base, which acts as support for a metallic lever, hk (fig. 387), movable in its middle on a horizontal axis. The extremity, B, of this lever is always pressed upwards by a spring, r, beneath; at the other

it works for instance in fig. 386. The current enters then by the wire, L, which is the line wire, rises into the lever, kh, and descends by the screw pin, a, into the wire, A, which leads to the indicator. If, on the other hand, the key is to be used for sending a message, as represented in fig. 385, it will be seen that the lever, kh, does not touch the metal pin in which the wire, P, terminates. But if the lever, h, is lowered by pressing the end, B, contact is set up, and the current, P, at once passes into the lever, hk, and thence into the wire, L, which leads it to the post signalled to; for the same wire is used to send and to receive the message.

The indicator consists of an electromagnet, E, which whenever

the current is transmitted, acts attractively on an armature of soft iron, m, fixed at the end of a lever, mn, movable about an axis, r; when the current is open, the lever is raised by a spring, R. At

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Telegraph Alphabet.

495 the other end of the lever there is a pencil, x, which writes the signals. For this purpose a long band of strong paper, ab, rolled round a drum, S (figs. 385 and 386), passes between two copper rollers with a rough surface, turning in contrary directions. Drawn in the direction of the arrows, the band of paper becomes rolled on a second drum, Q, which is turned by hand. A clockwork motion placed in the box, V, works the rollers, between which the band of paper passes.

The paper being thus set in motion, whenever the electromagnet works, the point, x, strikes the paper, and, without perforating it, produces an indentation, the shape of which depends on the time during which the point is in contact with the paper. If it only strikes it instantaneously, it makes a dot (.); but if the contact has any duration a line of corresponding length is produced. Hence, by varying the length of contact of the transmitting key at one station, a combination of dots or points may be produced at another station, and it is only necessary to give a definite meaning to these combinations.

This is effected as follows in Morse's alphabet:

Any one present while a message is being received at a telegraph station, is astonished at the promptitude and accuracy with which signals are read and transmitted by the operators. These acquire such skill that they can read a message by the sounds which the armature makes in striking against the electromagnet of the indi

cator.

474. Improvements in Morse's telegraph.-In the apparatus just described, the indentations on the paper only give indistinct dots and dashes, unless the current transmitted be very powerful. To get rid of this inconvenience, and to expend less force, the apparatus has been modified so that the signals can be traced in ink. With this view, all the other parts being the same, the following arrangement is made :

A roller, a, covered with flannel, is moistened with a suitable ink. Above the roller, and in contact with it, is an endless band rolled on two pulleys, o, o', which are turned by the clockwork motion which moves the paper. This is kept within a roller, b, very near the chain, but not touching it. That being premised, whenever the current passes in the electromagnet, the armature, A, is attracted, the arm of the lever, k, is depressed, and a pin, i, at its end rests on the band, and places it in contact with the paper. The band depositing the ink which it has taken from the roller, makes on the paper as it moves along, a dot or a dash, according to the length of time the current passes, and which dots and dashes have the same meaning as above.

475. Lightning conductor.-Besides the parts of the telegraph

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Electrical Alarum.

497

already described there are three of which mention must be made; the lightning conductor, the alarum, and the relay.

The influence of storm clouds in decomposing the natural electricity of the wire, often produces sufficient tension, not merely to interfere with the transmission of the despatches, but also to produce dangerous discharges. The lightning conductor is designed to remedy these inconveniences.

Represented at M in figs. 385 and 386, it consists of a vertical stand on which are two copper plates, indented like a saw, and arranged so that the teeth are near each other but do not touch. One of these plates is connected with the earth, the other with the line wire. Hence, when, by the inductive action of a storm cloud, electricity accumulates in wires and in the apparatus, it escapes by the points to the plate which is connected with the ground, and thus all danger from a discharge is avoided.

476. Electrical alarum.-The electrical alarum is intended to warn the receiving station that a despatch is about to be sent. Represented in fig. 391, it con

sists of a board on which is fixed an electromagnet by means of a piece of brass, E. The current from the line arriving by a binding screw, m, passes to the wire of the electromagnet, thence into the armature, a, into a steel spring, c, which presses against the armature, and ultimately emerges by a second terminal, n.

Thus, whenever the current of the line wire reaches the electromagnet, the armature, a, is attracted, and a clapper, P, fixed to this armature, strikes against a bell, T, and makes it sound. The moment the clapper strikes, as the armature is no longer in contact with the spring, C, the current is open, the electromagnet no longer attracts, and the armature reverts to its original position by the action of a spring, e, to which it is fixed. The current being closed afresh, a second attraction takes place,

K K

Fig. 391.

and so on until the telegraph clerk, thus warned, lets the current pass directly into the indicator without passing through the alarum. This he accomplishes by means of an instrument called the shunt.

Relay. In describing the receiver we have assumed that the current of the line coming by the wire, C (fig. 392), entered directly into the electromagnet, and worked the armature, A, producing a despatch; but when the current has to traverse a distance of a few miles, owing to the resistance of the wire and the losses of insulation, its intensity is diminished so greatly that it cannot act upon the electromagnet with sufficient force to print a despatch. Hence it is necessary to have recourse to a relay, that is, to an auxiliary electromagnet, which is still traversed. by the current of the line, but which serves to introduce into the communicator the current of a local battery of four or five elements placed at the station, and only used to print the signals transmitted by the wire.

For this purpose the current from the line entering the relay by the binding screw, L (fig. 392), passes into an electromagnet, E, whence it passes into the earth by the binding screw, T. Now, each time that the current of the line passes into the relay, the electromagnet attracts an armature, A, fixed at the bottom of a vertical lever, p, which oscillates about a horizontal axis.

At each oscillation the top of the lever, p, strikes against a button, n, and at this moment the current of the local battery which enters by the binding screw, c, ascends the column, m, passes into the lever, p, descends by the rod, o, which transmits it to the binding screw, T: thence it enters the electromagnet of the indicator, whence