Part 67

Below the dial of the instrument, in Fig. 282, may be seen two handles. Each of these is connected with an arrangement constituting the transmitting apparatus, by which the metallic contacts are varied according to the position of the handles. When the handle is vertical, all communication with the battery in connection with the instrument is cut off, but the coils are ready to receive any current from the line-wires. When the handle is turned to the right or left, the contacts are such that the battery current flows into the line, and deflects to the right or left the needles of both receiving and transmitting instruments. The single-needle instrument as now made is of a very simple and inexpensive construction, and it is the form principally used in connection with the working of lines of railway. One may see at every station in the United Kingdom the little vertical needle, mounted in the centre of a small perfectly plain green dial-plate; for the letters and signs with which it was formerly the practice to cover the dial have been found to distract the eye more than they aid the memory. A boy will after a few weeks’ practice learn to read the signals and to transmit messages with considerable rapidity.

The field telegraph lines, which are used in actual warfare to enable the commander of an army to communicate with every part of his forces, require as the essential condition for their construction rapidity of erection and removal, and the greatest possible simplicity and portability in the sending and receiving instruments. The wires are fastened to trees, or other fixed supports, where such are available, but artificial supports are provided in light poles which admit of being readily planted in the ground and removed. In cases where it is inexpedient or impossible to use these, the conductor may be laid along the ground, but must then be well insulated with some non-conducting material, which is capable of withstanding the action of the weather. A kind of cable is usually employed, in which is the conductor, made of copper, protected and strengthened by hemp fibres and covered with some non-conducting material. No form of needle telegraph instrument could be simpler than that represented in Fig. 284, which has been designed for military purposes. The communicator, or transmitting apparatus, here shows an arrangement very compact, and not easily deranged. The springs, A B, press against the piece of metal marked C, with which good contact is insured by providing the springs with several projecting steel points. D, E are finger-keys of ebonite or ivory; underneath are two points of a metallic conductor on which the springs can be pressed down by a touch of the finger. This conductor is in communication with the binding-screw, F, from which a wire proceeds to the negative or zinc end of the battery, while the piece, C, is in metallic connection with G, to which a wire proceeding to the positive or copper end of the battery is attached. From B a wire, H, communicates through the hinge with one end of the coil, the upper end of which is connected through the upper hinge with a binding-screw not visible in the figure, and to this the end of the line conductor is attached. From A a wire K passes to another binding-screw, by which the earth connection is made. A current arriving by the line traverses the coils and passes through H and B into C, hence by A into the earth through K. When D is depressed the current from the battery passing from G through C, A, and K, into the earth, and thus to the distant station, returns through the coils of the instrument there and along the line wire, through the coils, L L, and by H, B, D and F, to the negative pole of the battery. The reader will have little difficulty in tracing the course of the reverse currents, whether sent or received, which deflect the needles in the opposite direction.

The field telegraph instrument selected by the War Department of the United States Government is also extremely simple, communicating its signals, not by the deflections of a needle, but by the blows on an electro-magnet of its armature. The letters are indicated by various combinations of two signals—one, a single stroke of the armature; and the other, two blows in very rapid succession. The alphabet used is the “General Service Flag Code” of the American army and navy, and the signal numerals of this code are indicated by contacts of the transmitting key—one contact producing a single blow of the armature, implying the numeral 1, and two rapidly succeeding contacts causing two blows, which stand for the numeral 2. The signals are read merely by the sound made by the stroke of the armature. In the table below the code is given, dots being used to represent the contacts of the key in the “sending” instrument, and the blows of the armature in the “receiving” instrument—the single dots standing for one contact or sound, and the double dots for the double blows:

┌────────┬──────────┬────────────────────┐ │Letters.│Flag Code.│ Telegraph Signals. │ ├────────┼──────────┼────────────────────┤ │ A │2 2 │·· ·· │ │ B │2 1 1 2 │·· · · ·· │ │ C │1 2 1 │· ·· · │ │ D │2 2 2 │·· ·· ·· │ │ E │1 2 │· ·· │ │ F │2 2 2 1 │·· ·· ·· · │ │ G │2 2 1 1 │·· ·· · · │ │ H │1 2 2 │· ·· ·· │ │ I │1 │· │ │ J │1 1 2 2 │· · ·· ·· │ │ K │2 1 2 1 │·· · ·· · │ │ L │2 2 1 │·· ·· · │ │ M │1 2 2 1 │· ·· ·· · │ │ N │1 1 │· · │ │ O │2 1 │·· · │ │ P │1 2 1 2 │· ·· · ·· │ │ Q │1 2 1 1 │· ·· · · │ │ R │2 1 1 │·· · · │ │ S │2 1 2 │·· · ·· │ │ T │2 │·· │ │ U │1 1 2 │· · ·· │ │ V │1 2 2 2 │· ·· ·· ·· │ │ W │1 1 2 1 │· · ·· · │ │ X │2 1 2 2 │·· · ·· ·· │ │ Y │1 1 1 │· · · │ │ Z │2 2 2 2 │·· ·· ·· ·· │ └────────┴──────────┴────────────────────┘

There are similar signals for the numerals and for a few often-recurring syllables.

The telegraphs we have hitherto described leave no record of the despatches sent, and hence the messages cannot be read at leisure, and errors which may occur in the transmission cannot be traced to their source. A system which registers the messages as actually received has plainly many advantages over those which merely give a visible or audible signal without leaving any trace. Hence many contrivances have been proposed for making the receiving apparatus print the message in ordinary characters. Such instruments are necessarily very much more complicated in their construction than those we have already mentioned, and by no means so simple as the system we are about to describe, namely, the Morse Telegraph, which is now so largely used, being universally adopted in America and on the continent of Europe; and, since the telegraphic communication in Great Britain came into the hands of the Post-office authorities, here, also, the Morse is the system most approved.

The general arrangement of the transmitters, batteries, receiving instruments, &c., should be first studied in its simplest form, as represented by the diagram, Fig. 285. M represents the vertical coils of an electro-magnet upon which we are supposed to be looking down; the armature, A, is attached to a lever, F, which, by the attraction of the electro-magnet is therefore drawn down. In the position of the connections, as represented, no current is passing, but if K be pressed down so as to make connection at 1, at the same time it is broken at 2, a current will pass in from the positive pole of battery, B, into the line by 1, 3, L, L´, and through 3´, 2´ through the coils of the electro-magnet at M´ into the earth, and so back to the negative pole, Z. The armature, A´, will be attracted so long as the current continues. Similarly, contact made at 1´ and broken at 2´, will affect the electro-magnet, M, from the battery at B´. It should be noticed here that it is not a question of the reversal of currents sent from the same battery; the key merely enables the operator to send a current in one direction, so as to affect the distant electro-magnet whenever or so long as he depresses the key. We shall now examine the construction of the Morse receiving apparatus, one of the most complete forms of which is depicted in Fig. 286. In the present description we wish the reader to consider only the portion of the apparatus towards the left, and to suppose the absence of the electro-magnet at the right-hand side, with all the appliances immediately connected with it. He must regard the electro-magnet, A, as corresponding with M´ in Fig. 285, and remember that it is in the power of the distant operator at K to throw the current of his battery through the coils of A, by simply depressing his key. When the current passes the armature, B, it is attracted, and the lever, C, to which it is attached, turns on its bearings at D, and the end, E, of its longer arm is pressed upwards. At this end of the lever, in the earlier form of the instrument, was a blunt steel point which, while the armature was attracted to the electro-magnet, was pressed into a shallow groove in a metallic roller. Between the roller and the steel point a paper ribbon, half an inch wide, K, was unwound from the drum, L, by the two rollers, M and N, which grip the paper between them as they are turned by clockwork within the case, F.

An important improvement was effected when, instead of steel points for embossing the message, the Morse instrument was provided with an arrangement for printing the signals in ink; since the pressure required for embossing the paper is considerably greater than that needed merely to bring it into contact with the edge of a little inked disc. In the inking arrangement the strip of paper travels just below the margin of a vertical disc, turned by the clockwork, and having its plane parallel to the length of the paper strip. The narrow edge of this disc is kept charged with printer’s ink, which it receives from a roller. The end of the lever connected with the armature of the electro-magnet is formed of a light strip of metal carrying a narrow projection at the end, over which the paper passes, just beneath, but not touching, the inking disc. When the current passes, the little projection is lifted up, and raises the paper into contact with the ink, printing either a dot or a dash according to the duration of the current. The amount of force required to raise an inch or two of the length of the paper ribbon through a space not greater than the twentieth of an inch is but small, and much less than would be required to emboss the paper; so that in a great many cases the part of the apparatus which is represented in Fig. 286, on the right, may be dispensed with. In other cases it is, however, necessary; as when, from the length of the line, the currents are too feeble to give clear indications with the printing lever; and we shall, therefore, presently describe its arrangement and purpose.

The clockwork is actuated by a spring, wound by the handle G, but its action is suspended by a detent, which is released by touching the lever H. When the clockwork is in action and the current constantly circulating in the coils, a continuous line, parallel to the length of the ribbon, would be printed upon it, in consequence of the contact with the inking-disc, P, being maintained; but when a momentary current only rushes through the coils, the armature attracted but for an instant, gives rise to merely a dot on the passing paper, while a current of a little duration will cause the paper to be marked with a short line or dash.

The dot and the dash are the elementary signs of the Morse code of signals, and these are producible according to the time the contact key is held down at the distant station. By employing various combinations of these two signs, the letters of the alphabet, numerals, &c., are indicated. In selecting the combinations Professor Morse had regard to the frequency with which the different letters recur in the English language. Thus, for the letter E, which is more frequently used than any other, the symbol chosen was a single dot; and for T, which is the next most frequently employed, the dash was plainly the most appropriate; then the four only possible combinations of the signs in pairs fell to the next most frequent letters, and so on. The following table gives the complete Morse code. The eye of the reader will doubtless detect a kind of symmetry in the arrangement of the signs for the first five and last five numerals:

ALPHABET.

┌──────────────────────────┬─────────┐ │ Letter. │ Sign. │ ├──────────────────────────┼─────────┤ │ A │·- │ │ Ä │·-·- │ │ B │-··· │ │ C │-·-· │ │ D │-·· │ │ E │· │ │ É │··-·· │ │ F │··-· │ │ G │--· │ │ H │···· │ │ I │·· │ │ J │·--- │ │ K │-·- │ │ L │·-·· │ │ M │-- │ │ N │-· │ │ O │--- │ │ Ö │---· │ │ P │·--· │ │ Q │--·- │ │ R │·-· │ │ S │··· │ │ T │- │ │ U │··- │ │ Ü │··-- │ │ V │···- │ │ W │·-- │ │ X │-··- │ │ Y │-·-- │ │ Z │--·· │ │ Ch │---- │ └──────────────────────────┴─────────┘

NUMERALS.

┌──────────────────────────┬─────────┐ │ Numeral. │ Sign. │ ├──────────────────────────┼─────────┤ │ 1 │·---- │ │ 2 │··--- │ │ 3 │···-- │ │ 4 │····- │ │ 5 │····· │ │ 6 │-···· │ │ 7 │--··· │ │ 8 │---·· │ │ 9 │----· │ │ 0 │----- │ └──────────────────────────┴─────────┘

PUNCTUATION, &c.

┌──────────────────────────┬─────────┐ │ │ Sign. │ ├──────────────────────────┼─────────┤ │Full stop │······ │ │Colon │---··· │ │Semicolon │-·-·-· │ │Comma │·-·-·- │ │Interrogation │··--·· │ │Exclamation │--··-- │ │Hyphen │-····- │ │Apostrophe │·----· │ │[6]Fraction-line │------ │ │[7]Inverted commas │·-··-· │ │[7]Parenthesis │-·--·- │ │Italics or underlined │··--·- │ │New line │·-·-·· │ └──────────────────────────┴─────────┘

Footnote 6:

To be placed between the numerator and denominator of a vulgar fraction.

Footnote 7:

To be placed before and after the words to which they refer.

OFFICIAL SIGNALS.

┌──────────────────────────┬─────────┐ │ │ Sign. │ ├──────────────────────────┼─────────┤ │Public message │··· │ │Official Telegraph message│·- │ │Private message │·--· │ │Call │-·-·-·- │ │Correction, or rub out │···-· │ │Interruption │·········│ │Conclusion │·-·-·-· │ │Wait │·-··· │ │Receipt │·-··-··-·│ └──────────────────────────┴─────────┘

The length of a dot being taken as a unit, the length of a = 3 dots. dash

The space between the signs composing a letter = 1 dot.

The space between two letters of a word = 3 dots.

The space between two following words = 6 dots.

Fig. 287 is a view of the Morse transmitting key. A B is a brass lever, moving in bearings at C, and provided at the end of its longer arm with a large knob or button of some insulating material. Steel pins are screwed in at B and D, and they are so adjusted that while that at B is pressed against the projection, E, by the action of the spring, F, when the knob, K, is pressed, contact is broken at B, and established at D. D and E are each provided with a binding-screw, so that wires may be attached in the manner indicated in Fig. 285. When the key is in the position shown, a current arriving by the line-wire passes from the fulcrum, C, of the lever through the contacts into the apparatus. When the knob is pressed down the battery current enters the lever by the contact at D, and passes into the line from the fulcrum, C. The clerks who are called upon to transmit messages usually soon learn to time the contacts very accurately in accordance with the code of signals, so as to produce the dashes and lines with accuracy. However, with certain persons some difficulty was found in acquiring the requisite uniformity, and to obviate any objection on this score, Morse invented an arrangement for facilitating the signalling, which is represented in Fig. 288. This is a smooth tablet of a non-conducting substance, such as ivory, except the shaded portions, which are plates of metal having their surfaces even with that of the ivory, and all soldered to a plate of metal beneath the ivory, which places them all in communication with each other and with the binding-screw, C. The lengths of the strips of metal and those of the spaces between them correspond with the dots and dashes of the Morse alphabet as marked on the tablet. The battery wire is connected with the binding-screw, C, and the line-wire terminates in an elastic and flexible coil of insulated wire, which is attached to a short rod having an insulated handle and terminated by a blunt platinum point. This the transmitter takes in his hand and draws uniformly along the line of metal strips belonging to the letter which he wishes to telegraph. The circuit is closed while the point of the style is passing across the metallic strips. This arrangement appears to be but little used, but it is nevertheless admirable for its simplicity, and is described here as a good illustration of the mode in which the varied duration of the contacts is able to produce the signals of the Morse alphabet. With the ordinary transmitting key a clerk is able to telegraph, on the average, twenty or twenty-five words in a minute, but the receiving apparatus is capable of recording three times as many. Morse also invented a system of transmitting the messages automatically, by setting up the message in a kind of type, just as ordinary letters are arranged for printing. The type, if it may be so called, had simple projections like the slips of metal, corresponding with each letter in Fig. 288. The lines of the message were drawn under a contact-lever, which closed the circuit when lifted up by the projections. Thus the speed of transmission could be very greatly increased, and a single wire and apparatus had its capacity of conveying a great number of messages in a given time proportionately enlarged.

We have now to ask the reader’s attention to the details of the apparatus in Fig. 286, the use of which has not already been pointed out. The electro-magnet, O O´, and the parts immediately connected with it, form what is called a _relay_. The object of this may be illustrated by supposing that the instrument is at one end of a long line, such as that between Edinburgh and London. Let us suppose it is at Edinburgh: the currents sent from London by a battery of convenient size might not be powerful enough to magnetize the soft iron of A with sufficient intensity to give clearness to the signals. They are, therefore, made to circulate in the electro-magnet, O, where they act by attracting the armature, W, which has the form of a split tube of soft iron, attached to a very light lever, Q, adjusted with great delicacy, and so that it moves by little magnetic force. The end of the lever works between two adjustable screws, R and S, which are electrically insulated, except that R is in communication with one extremity of the coils of the electro-magnet, A. Q is in metallic communication through the pillar, T, and the binding-screw, U, with the zinc end of a battery at Edinburgh, which is called the local battery, the other pole of which communicates with the other ends of the coils, A, through the screw, U´. When no current from London is passing through O, Q is held down by the spring, W´, and the circuit of the local battery is broken; but the instant the line-current passes, the armature, W, is attracted, and Q makes contact with R, the current from the local battery rushes through the coils, A, and the appropriate movements of the printing lever are effected by its action. X is a spring for drawing down the lever, and it is provided with a screw for adjusting its tension, and Y, Z, are screws for limiting the extent of motion of the lever; under P is the little projection by which the band of paper is pressed against the inking-disc; _l_ and _e_ are respectively the screws for the line and earth connections.