Fig 252.—Passage of the current (1).
Fig. 253.—Passage of the current (2).
For the sake of clearness, the diagram has been drawn with simple lines only. In the real needle-machine the construction is much more complicated; perspective drawings of it may be seen in Lardner’s “Electric Telegraph,” and numerous other works. In fig. 1, B is a single cell of a battery containing a plate of copper, C, and a plate of zinc, Z, immersed in sulphuric acid and water. H is the handle of the instrument, turning from left to right, and vice versâ, like the handle of a door, consisting of two pieces of brass insulated from each other by being inserted in an axis of ivory. To the ends of the two pieces of brass are fixed the wires, CW and ZW, leading from the copper and zinc ends of the cell respectively. Fixed on each side of the handle are two plates of metal, which may be called LP, the left plate, and RP, the right plate. They are connected with the needle wire, NW, which passes before and behind the magnetized needle, N, suspended perpendicularly on its axis, with its north pole upwards. As long as the wires, CW and ZW, remain insulated from each other, no current passes from the cell; but as soon as the handle, H, is turned, so that the copper end touches the left plate (fig. 2), and the zinc end touches the right, communication is established between the plates of the cell, and the current commencing at the copper passes along CW, the top half of H, into LP, along N W, travelling up before and down behind the needle, causing it to deflect to the observer’s left, according to the rule given above. Reaching RP, it passes downwards to the cell to Z, and so on to C, continuing its travels and keeping the needle deflected as long as the handle remains in contact with the plates. If it is required to deflect the needle to the right, the handle is turned to the right, bringing its copper end in contact with the right plate, causing the current to travel in the opposite direction. By following the current from the copper to the zinc, as indicated by the arrows in fig. 3, it will be seen that it now travels up behind and down before the needle, deflecting it to the observer’s right. Thus, by causing a current of voltaic electricity to pass alternately up before and down behind, and up behind and down before, the needle is moved to the left or the right at will. The way in which the current is made to act on a distant needle is now simple. The following figure (fig. 253) shows the arrangement. The left portion of the figure represents an instrument at London, that on the right an instrument at York. The needle-wire, instead of being continued directly to the zinc plate of the battery, passes away from the needle over poles to York, where it joins an instrument similar in all respects to that at London. It passes similarly before and behind a magnetized needle, joining the right plate of the instrument. As long as the two plates are unconnected, no current can pass. The current is therefore completed by a contrivance which may be represented by the semicircular piece of metal, K. In practice the two plates or springs, which, when not in use, are always pressed against the connector, K, which is a cross-piece on the top of the handle, keeping the London needle in circuit with the York battery, and vice versâ. As soon as London uses his handle, it presses the spring-plate, and puts his needle out of the York circuit, the current he sets up sending York needle to the right or left, as the case maybe. The second wire connecting the left York plate with the right London plate is, it will be seen, not carried along like the first wire. Use is made in this case of the conducting power of the earth itself, plates from the wires being buried many feet below the surface at London and York. When London wishes to speak to York, he first signifies his intention of so doing by ringing York’s alarum. This he effects by sending a current through an electro-magnet placed above York’s instrument. The armature is attracted, and frees the detent of the alarum, setting it ringing until York signals ready. London then stops the bell, and commences his message. By following the direction of the current, when the handle is turned to the left, as in fig. 4[within fig. 253], it will be readily seen how this is effected: commencing with London’s copper, LC, it passes up before and down behind London’s needle, flowing along the wire between the two cities to York’s needle, up before and down behind which it travels, sending it also to the left. It then passes to York’s right plate, through the connector to the left plate, and so on to earth at York, coming to the surface again at London, passing through London’s right plate and through the lower part of the handle to the zinc of the battery. The reverse current may be easily followed. Any number of instruments with similar needles may be interposed along the course of the wire. When the operator wishes to speak to any particular one, he rings all the bells for attention, and then signals Derby or Nottingham, as the case may be. They all then throw their instruments out of current except the one required. The mode by which the needle movements are converted into language is simple. A is signalled by causing the needle to vibrate once to the right, B once right and once left, C once right and twice left; and so on, as arranged.
The following is the alphabet (with numbers) once in use on the South-Eastern Railway for the double-needle instrument. The table is taken from Mr. Walker’s translation of De la Rive’s work on Electricity and Magnetism.
| A | is signified | by two | movements of | left needle | to left. |
| B | ” | by three | ” | ” | ” |
| C | (and fig. 1) | by two | ” | ” | right first, then left. |
| D | (and fig. 2) | by two | ” | ” | left first, then right. |
| E | (and fig. 3) | by one | ” | ” | to the right. |
| F | by two | ” | ” | ” | |
| G | by three | ” | ” | ” | |
| H | (and fig. 4) | by one | ” | right needle to the left. | |
| I | by two | ” | ” | ” | |
| J | (same as G). | ||||
| K | by three | ” | ” | ” | |
| L | (and fig. 5) | by two | ” | ” | right and left. |
| M | (and fig. 6) | by two | ” | ” | left and right. |
| N | (and fig. 7) | by one | ” | ” | to the right. |
| O | by two | ” | ” | ” | |
| P | by three | ” | ” | ” | |
| Q | (same as K). | ||||
| R | (and fig. 8) | by one parallel movement of lower points, both needles to the left. | |||
| S | by two | ” | ” | ” | |
| T | by three | ” | ” | ” | |
| U | (and fig. 9) | by two | ” | ” | first right, then left. |
| V | (and fig. 0) | by two | ” | ” | first left, then right. |
| W | by one movement of both needles (lower points) to right. | ||||
| X | by two | ” | ” | ” | |
| Y | by three | ” | ” | ” | |
| Z | (same as S) or specially. | ||||
The Morse system of telegraphy was first brought out in 1844, and was worked by means of a Voltaic battery, an electro-magnet being used at the receiving station. This magnet attracted an “armature,” and by it dots or lines are marked on a moving paper band by a point at the other end of the wire, on the register in which the paper is carried by rollers which move out by clockwork. The lever being “tapped” down in fast or slow pressures will give a corresponding series of dots or lines (according as the pressure is long or short) upon the moving strip of paper at the receiving station. Three taps will give C, one tap and a pause will make A. The dots are “taps” on the key, the lines brief “rests” on it, as will be seen from the alphabet below, which is given as a specimen.
| Morse Alphabet. | |||
|---|---|---|---|
| A .- | G —. | M — | S ... |
| B -... | H .... | N -. | T - |
| C .. . | I .. | O . . | U ..- |
| D -.. | J -.-. | P ..... | V ...- |
| E . | K -.- | Q ..-. | W .— |
| F .-. | L - | R . .. | X .-.. |
| Y .. .. | Z ... . | ||