pass, and to the right when the "dash," or negative, currents pass. The play of the needle is confined by stops, so that it may not move too far, and the clerk watching its movements interprets the message. The sending key for these "double current" signals is a double form of the Morse "single current" key, which we have already noticed. It has two levers instead of one, and is so connected between the line and battery that when one lever is pressed by the finger of the

L

E

N

P

FIG. 22.

operator, a positive current enters the line, and when the other lever is pressed a negative current enters. Thus, in Fig. 22, L and E are the two levers, L being connected to the line wire, and E to the earth-plate. The two poles of the battery (positive and negative) are connected to the crossbars c and z respectively.

Now, when the lever L is pressed down by means of the finger cup N, the line is connected to the negative pole of the battery, and the positive pole is joined to earth through the bar c and the lever E, which presses up against it. The result is, that a negative current traverses the line and actuates the needle instrument. On the other hand, when the lever E is pressed down the negative current is put to earth, and the positive current enters the line and works the receiver at the other end.

Although the double current method of telegraphing is not much employed on land circuits, it is largely

used on submarine cables, in conjunction with a very delicate modification of the single needle instrument, known as the mirror galvanometer. The single needle instrument is chiefly to be found on railways at this day, or in some out-of-the-way telegraph offices where the traffic is very slight. It is a survival of Messrs. Cooke and Wheatstone's original instruments, and is sometimes worked according to a signal code of its The double needle instrument is a modification of it, in which there are two coils and two needles, the signals being made by operating both. Latterly, too, the visible signals of the needle instrument have been turned into audible ones by making the stops capable of giving out a ringing sound, as we have already said is the idea of Sir Charles Bright's bell sounder, in which the oscillating needle hits two bells of different pitch.

own.

An expert clerk can only transmit about twenty-five words per minute, because each average English word of five letters requires about fifteen separate dots and dashes to signal it. But by means of Wheatstone's Automatic Sender as many as a hundred and fifty words per minute may be telegraphed. In this instrument, which is too complicated to describe within our limits, the sending is done by a strip of paper on which the message is represented by three rows of holes, punched out by the punching clerk. Every hole in the right hand row is a dot, and every hole on the left hand row is a dash, while the central holes are merely to pull the paper through the sender. The strip of paper therefore contains the words of the message, much in the same way as the cards of the Jacquard loom comprise the pattern of the woven

fabric. On being passed through the sender two little plungers in connection with the poles of the battery slide along the surface of the paper and dip through the holes one after another, at each dip making contact with the line wire and sending a current into it. For every hole on the right side a positive current is sent, and for every hole on the left a negative current. The Morse ink-writer is well adapted for receiving messages sent by the automatic transmitter, and the latter is modified so as to send the long and short single current signals required.

The duplex system, whereby two messages, one in either direction, can be sent on one wire simultaneously, and the quadruplex system, whereby four messages, two in either direction, are sent, both depend upon ingenious ways of connecting up the receiving instruments at the end of the line, so that the signal currents sent out from a station do not affect the receivers there, while those which arrive at the station do. This plan will be understood from Fig. 23, which represents the arrangement at each station. As before, B is the battery supplying the current, K is the sending key for interrupting it; but instead of joining the end of the line L direct to the middle of the key, it is connected through a "resistance," or coil of wire R. Another coil of wire of equal resistance R' also branches from the middle of the key at the point a, and is connected to one end of an "artificial line" A L, equivalent in all electrical respects to the actual line. The other end of this mimic line is connected to the earth-plate, together with the other pole of the battery. The receiving instrument & is connected between the home ends of the line and the artificial

R R'.

line by a wire bridging across the branched resistance Now it will be readily understood that on pressing down the sending key the current from the battery will split at A, and part will flow to earth

through the telegraph line L, and part to earth through the artificial line A L. But since the artificial line is just equal to the real one, and the resistance R, is equal to the resistance R', the two portions will be equal to each other, and one half the current will flow by each route. The result will be that the receiving instrument will remain unaffected by the outgoing currents; while at the same time the disturbance of this electric "balance" by currents coming in from the other station will affect it and cause it to make signals. Thus each station can both send and receive a message at the same time. This method of duplexing was first successfully applied on land lines by Mr. J. B. Stearns, of New York; and in submarine cables by Dr. Alexander Muirhead. The quadruplex system is a

combination of the duplex with a plan for sending two messages simultaneously in the same direction along a wire. This is effected by sending two currents of different strengths, each capable of working a separate receiver. It was practically introduced for the first time by Messrs. Edison and Prescott, of America.

These two American systems have been adapted into the English postal telegraphs, and they are sometimes worked in conjunction with the Wheatstone sender when there is a glut of press news. As many as four hundred words per minute can then be sent over a single wire. The most ingenious multiplex system now in use is the Harmonic Telegraph of Mr. Elisha Gray, of Chicago, by which five independent messages are sent together in the same direction along one wire. This is done by five tuning-forks of different pitch, each vibrating and interrupting the circuit as it vibrates. In this way rapid pulses of current are sent into the line, and these are caused by electro-magnets at the receiving-station to set five corresponding tuning-forks in vibration there. Only the pulses due to the sending tuning-fork which has the same pitch will affect a particular receiving-fork. Now, if the five vibratory currents are broken into "dots" and "dashes" by five sending-keys, the five corresponding notes given out by the receiving-forks will also be broken into audible signals. Mr. Gray, however, does not stop at mere sound, and by means of local batteries and Morse ink-writers causes the tuning-forks to record their "hums" and "pauses" on a strip of paper as dots and dashes.

Another multiplex system, successfully employed in France, is that of M. Meyer, and consists in putting