The duration of the current is at the will of the operator who works the sending-key, and it is plain that signals can be made by currents of various lengths.  In the “Morse code” of signals, which is now universal, only two lengths of current are employed—­ namely, a short, momentary pulse, produced by instant contact of the key, and a jet given by a contact about three times longer.  These two signals are called “dot” and “dash,” and the code is merely a suitable combination of them to signify the several letters of the alphabet.  Thus e, the commonest letter in English, is telegraphed by a single “dot,” and the letter t by a single “dash,” while the letter a is indicated by a “dot” followed after a brief interval or “space” by a dash.

Obviously, if two kinds of current are used, that is to say, if the poles of the battery are reversed by the sending-key, and the direction of the current is consequently reversed in the circuit, there is no need to alter the length of the signal currents, because a momentary current sent in one direction will stand for a “dot” and in the other direction for a “dash.”  As a matter of fact, the code is used in both ways, according to the nature of the line and receiving instrument.  On submarine cables and with needle and “mirror” instruments, the signals are made by reversing currents of equal duration, but on land lines worked by “Morse” instruments and “sounders,” they are produced by short and long currents.

The Morse code is also used in the army for signalling by waving flags or flashing lights, and may also be serviceable in private life.  Telegraph clerks have been known to “speak” with each other in company by winking the right and left eye, or tapping with their teaspoon on a cup and saucer.  Any two distinct signs, however made, can be employed as a telegraph by means of the Morse code, which runs as shown in figure 46.

The receiving instruments R R’ may consist of a magnetic needle pivotted on its centre and surrounded by a coil of wire, through which the current passes and deflects the needle to one side or the other, according to the direction in which it flows.  Such was the pioneer instrument of Cooke and Wheatstone, which is still employed in England in a simplified form as the “single” and “double” needle-instrument on some of the local lines and in railway telegraphs.  The signals are made by sending momentary currents in opposite directions by a “double current” key, which (unlike the key K in figure 45) reverses the poles

    A .- J -.-. 
    B -...  K -.-
    C ...  L —­
    D -..  M — —
    E .  N -. 
    F .-.  O . . 
    G —.  Q ..-. 
    H . ..  R . .. 
    I ..

S ... 1 .—.  T — 2 ..-..  U ..- 3 ...-.  V ...- 4 ....- W .- 5 —–­ X .-.. 6 ......  Y .. .. 7 —..  Z ... . 8 -.. .. & . .. 9 -..- Period ..—.. 0 ——­ Comma .-.-

The International (Morse) code used elsewhere is the same as the above, with the following exceptions: