potential), since each has a large surface in contact with a practically unlimited conductor. And, as long as each end of the line is kept at zero potential (or the same potential), so long will the battery send a current through it.

(1) The electric bell.-The accompanying figure represents a very simple form of signalling instrument: it is similar in principle to the automatic make-and-break de

scribed in Chapter XXII. § 7.

When in the position shown here (see fig. i.) the circuit is complete. But, as soon as a current passes, the electromagnet attracts the iron keeper a, causing the hammer to strike the bell, and the circuit is broken at the place where the spring C touched the keeper a. The current ceases, and with it the magnetism. Hence the keeper a is no longer attracted; and so, acted upon by the spring to which it is attached, it flies back again and makes contact once more with C.

then repeated. As

The process is

long, therefore,

as the current is kept on the line, so

long will the hammer vibrate and the

FIG. i.

OP

bell sound. This instrument is used only for attracting attention,

not for messages.

a

b

FIG. ii.

(2) The electric sounder. This instrument (see fig. ii.) is very simple in construction. The current from the sending station passes

EE

through the coils of the electro-magnet, and the iron keeper a is attracted downward. This causes the other end of the lever to hit with a sharp sound the screw d. When the current ceases, the spring S pulls the lever down again, and this hits the lower screw e. When the current is made and broken very rapidly, these two sounds occur almost simultaneously, as a sharp double click.' If the current be not broken again at once,

the two sounds are separated by a corresponding interval, and the sound is more deliberate. These two sounds can be readily distinguished; and by suitable combinations of these we can, as will be explained in § 3, represent all the letters of the alphabet, and so form a code for messages.

It is to be noticed that if there be passing between the point d and the lever a strip of paper, moved by clock-work at a uniform

rate, then each sharp make-and-break of current will cause a dot to be made on the strip; while each slower make-and-break will keep the point pressed against the paper sufficiently long to make a dash on it. When the signals are thus to be printed, the instrument is modified, the

graph. By sending currents round any form of galvanometer in opposite directions, we can give to the needle deflexions in opposite directions respectively. On this principle is constructed the common needle telegraph,' of which one form is shown in fig. iii. Here the coils and the needle are both vertical, and the motion of the needle is limited by two ivory stops. In fig. iv. we see at the base of the instrument the commutating key by means of which the current can be made or broken, and sent in either direction. The current passes first through the sending instrument, deflecting its needle in one or the other direction. It passes thence through the line wire, and affects in a corresponding manner the needle of the receiving instrument.

FIG. iv.

By means of a suitable code both this and the last instrument are used to transmit verbal messages.

§ 3. Telegraphic Alphabets.-In all those instruments that are in extensive use there are only two elementary signals: the dot, to which we agree to consider a deflexion to the left to be the

corresponding signal in the needle instrument; and the dash, to which corresponds the deflexion to the right of the needle.

The signals answering to the letters of the alphabet are various combinations of the above elements, the only principle observed being that simple signals shall represent constant recurring letters.

This system is called, from its inventor, the Morse code. The reader will notice how readily this code, intended originally for dots and dashes in the Morse instrument described in § 5, adapts itself to other methods of signalling; as by the needle instrument, by the voice, by flashes of light, by flags, &c.

$ 4. The Needle System of Telegraphy.-We will now describe briefly the simplest manner in which two stations may be united on the needle system. In the figure, A and B represent the two needle instruments (see § 2, fig. iii.) at the two stations respectively; the batteries are represented as usual. E and E' are large plates buried in the earth, serving to keep the two ends of the line at the zero potential; K and K' are the two commutating keys by means of which each station can send a current in either

direction through the other station; the line wire (shortened out of all proportion, of course) is seen at the top of the diagram.

Action of the commutator key.-The terminals of the battery are connected with two strips of metal, marked white and black respectively in our diagram. These are insulated from each other. The pieces and are two keys of metal, that are kept pressed against the upper (or white) strip by means of a spring. In this,

A

B

E

K

Κ'

which is the normal condition of things, there is a complete circuit through A, the upper strip, the line, B, the other upper strip, the earth plate E', the other earth plate E, and so to A again. The batteries are cut out of the circuit; since the lower (or black) strips, and with them the negative poles of the batteries, are insulated.

'Now let the key / be depressed; thus leaving the upper, and touching the lower, strip. The connections make it evident that this will send a current along the line to B, through E' and E, and back to the pole of the battery by A and . If the other key be depressed, the current will pass in the other direction.

If similar keys be depressed at both stations simultaneously the circuit will be broken, and no message can be sent.

§ 5. The Morse System.-The Morse instrument is in all essentials the sounder of § 2, fig. ii. ; but it is modified so as to print dots or dashes on a strip of paper.

In the figure we show the simplest manner of connecting two stations by means of Morse instruments. The two Morses are at M and M'; K and K' are sinple keys, not commutators; G and G' are galvanometers to indicate to the sender whether or no the current actually passes, or whether the line is stopped; this being