Now, iu order that the clerk may have it in his power to send the current in either direction, so as to deflect the distant needle either to the right or to the left at pleasure, some such arrangement as the following is necessary. There are many arrangements, more or less simple, having the same object in view, but we give this one for the sake of illustrating the principle :-Fig.13 represents the galvanometer coil, with its needle, a 8, at rest, or in a position to receive a message. Now, suppose the current from the distant battery to enter the coil a by the wire L. It will pass through the coil and come out by the left-hand wire connected with a metallic spring, v. It will then pass along the brass cross-piece d to the metallic spring t, and complete the circuit by means of the wire attached to the earth-plate r, and the earth, by which it returns to the distant station. The battery c z, at the receiving station, remains inactive, its extreme wires being attached to insulated pieces of brass at either end of the vertical piece connected with d. Hence no current can pass, since the wire from c number of stations which can communicate with each other simul taneously, the larger must be the number of the wires.
When the instrument consists of two needles and two coils the alphabet may be made out somewhat in the following manner. Calling the left-hand needle No. 1, and the right-hand No. 2, and indicating one movement of either needle to the left by 4 and one movement to the right by r, the combinations of movements which stand for the various letters of the alphabet are the following : is insulated. If, however, a signal has to be transmitted from this instrument to the distant station, the clerk works the handle in front of the instrument, and presses ti against the spring 1, 14, whereby the lower extremity of the braes piece is brought into contact with the other spring, v, and the current now passes in the direction of the arrows, that is, from c to v, through the wire attached to P, into the earth through the distant station, where the instrument is arranged for receiving the signal, as in fig. 13, where it produces a deflection of the needle ; the current then returns by L to the galvanometer coil o, deflects the needle, and returns by the wire attached to the spring and so by the metallic piece d, through the wire attached to z, and thus completes the circuit. By reversing the motion of the handle, the direction of the current, and the motion of the needles in the coil, will be reversed, both in the near and in the distant instrument, as in fi j.15. When the clerk has finished making his signals, the springs v and restore the croas-pieco d to the position shown in Ai. 13, so that the instrument adjusts itself for receiving signals from the distant station, the battery o z is thrown out of action, and the conducting communi cation with the line is restored by means of the cross-piece d. In this way the same motion of the needle is produced at the same instant at both stations, so that the sender and the receiver of the message each perceives the signals.
There is in general but one needle in connection with each galvano meter coiL The aetatic arrangement formerly adopted is not now in use ; the outside needle, by which the clerk knows how the signals are proceeding, being formed of ivory, or some inert substance, attached to the same axis which carries the magnetic needle within the coiL The needles are limited in their motions by means of small ivory studs fixed in the dial, and they are thus prevented from swinging too far over, while their motions are made precise and clear. It was formerly the practice to give a preparatory notice of a message to the distant station by ringing the electric alarum, but this was found to be so intolerable to the persons in waiting that it has been abandoned, and the clicking sound of the needle against the ivory studs is sufficient to draw the attention of the clerk to the arrival or passing of a message. When a message is sent from London to York, for example, all the needles of the intermediate stations are similarly deflected ; but each clerk knows, ty a special signal, when he is required to be spoken with. There are, however, contrivances called " commutators," by which a message is made to pass any one or more stations without entering them. In this way intermediate stations may communicate with each other ; otherwise, they would have to remain idle while two exterior stations were communicating with each other. Of course, the larger the Of course it is possible to form an alphabet with one needle and one wire only ; the communication is not so rapid as with two wires and two needles, but the construction of the line is more economical, and the system is adapted for use between places of second-rate importance. Various alphabets and modes of signalling have been contrived with a view to celerity, and some of these are ingenious. We select ono by the Rev. H. }Lighten, as described by Mr. E. Highton : a small slip of gold leaf inserted in a glass tube is made to perform part of the electric circuit of the line wire, and near it is a permanent magnet. When a current of electricity is passed along the line wire, the gold leaf is instantly deflected to the right or to the left according to the direction of the current. Now supposing the deflection of the gold leaf to the left signified the figure 1, and the deflection to the right the figure 3, we have the alphabet. made up in the following manner : twice to the right, or 33, signifies A; twice to the left, once to the right, and once to the left, or 1131 =B; 311 = C ; 133 = D ; a single signal to the left, or 1=E ; thus acting on the correct principle of representing the letters of most frequent occurrence by the most rapidly executed signals ; F 313, 1133, H 113, 1 31, J 3133, K 1331, L 331, 31 1113, N 13, 0 11, P 1111, Q 1313, It 333, S 111, T 3, U 131, V 1311, W 1333, X 3113, Y 3111, Z 3131. A motion to the left signifies " Do under stand," and one to the right " Not understand." " Repeat " is ex pressed by 3331, and " Wait " by 3333.