C -.-.  Q —.-
    F . -.  R .-. 
    J .—–­ X -..-
    L .-..  Y -.—­
    O —–­ Z —.. 
    P .—.

    Fig. 46.—­Morse Signal Alphabet.

of the battery, in putting the line to one or the other, and thus making the “dot” signal with the positive and the “dash” signal with the negative pole.  It follows that if the “dot” is indicated by a throw of the needle to the right side, a “dash” will be given by a throw to the left.

Most of the telegraph instruments for land lines are based on the principle of the electro-magnet.  We have already seen (page 59) how Ampere found that a spiral of wire with a current flowing in it behaved like a magnet and was able to suck a piece of soft iron into it.  If the iron is allowed to remain there as a core, the combination of coil and core becomes an electro-magnet, that is to say, a magnet which is only a magnet so long as the current passes.  Figure 47 represents a simple “horse-shoe” electro-magnet as invented by Sturgeon.  A U-shaped core of soft iron is wound with insulated wire W, and when a current is sent through the wire, the core is found to become magnetic with a “north” pole in one end and a “south” pole in the other.  These poles are therefore able to attract a separate piece of soft iron or armature A. When the current is stopped, however, the core ceases to be a magnet and the armature drops away.  In practice the electromagnet usually takes the form shown in figure 48, where the poles are two bobbins or solenoids of wire 61 having straight cores of iron which are united by an iron bar B, and A is the armature.

Such an electromagnet is a more powerful device than a swinging needle, and better able to actuate a mechanism.  It became the foundation of the recording instrument of Samuel Morse, the father of the telegraph in America.  The Morse, or, rather, Morse and Vail instrument, actually marks the signals in “dots” and “dashes” on a ribbon of moving paper.  Figure 49 represents the Morse instrument, in which an electromagnet M attracts an iron armature A when a current passes through its bobbins, and by means of a lever L connected with the armature raises the edge of a small disc out of an ink-pot I against the surface of a travelling slip of paper P, and marks a dot or dash upon it as the case may be.  The rest of the apparatus consists of details and accessories for its action and adjustment, together with the sending-key K, which is used in asking for repetitions of the words, if necessary.