heroes of the telegraph-第18章
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admirable in its way。 The Hughes； or the Stock Exchange instruments； for instance； print the message in Roman characters； the sounders strike it out on stops or bells of different tone； the needle instruments indicate it by oscillations of their needles； the Morse daubs it in ink on paper； or embosses it by a hard style； while Bain's electro…chemical receiver stains it on chemically prepared paper。 The Meyer…Baudot and the Quadruple receive four messages at once and record them separately； while the harmonic telegraph of Elisha Gray can receive as many as eight simultaneously； by means of notes excited by the current in eight separate tuning forks。
But all these instruments have one great drawback for delicate work； and； however suitable they may be for land lines； they are next to useless for long cables。 They require a certain definite strength of current to work them； whatever it may be； and in general it is very considerable。 Most of the moving parts of the mechanism are comparatively heavy； and unless the current is of the proper strength to move them； the instrument is dumb； while in Bain's the solution requires a certain power of current to decompose it and leave the stain。
In overland lines the current traverses the wire suddenly； like a bullet； and at its full strength； so that if the current be sufficiently strong these instruments will be worked at once； and no time will be lost。 But it is quite different on submarine cables。 There the current is slow and varying。 It travels along the copper wire in the form of a wave or undulation； and is received feebly at first； then gradually rising to its maximum strength； and finally dying away again as slowly as it rose。 In the French Atlantic cable no current can be detected by the most delicate galvanoscope at America for the first tenth of a second after it has been put on at Brest； and it takes about half a second for the received current to reach its maximum value。 This is owing to the phenomenon of induction； very important in submarine cables； but almost entirely absent in land lines。 In submarine cables； as is well known； the copper wire which conveys the current is insulated from the sea…water by an envelope； usually of gutta…percha。 Now the electricity sent into this wire INDUCES electricity of an opposite kind to itself in the sea…water outside； and the attraction set up between these two kinds 'holds back' the current in the wire； and retards its passage to the receiving station。
It follows； that with a receiving instrument set to indicate a particular strength of current； the rate of signalling would be very slow on long cables compared to land lines； and that a different form of instrument is required for cable work。 This fact stood greatly in the way of early cable enterprise。 Sir William （then Professor） Thomson first solved the difficulty by his invention of the 'mirror galvanometer；' and rendered at the same time the first Atlantic cable company a commercial success。 The merit of this receiving instrument is； that it indicates with extreme sensibility all the variations of the current in the cable； so that； instead of having to wait until each signal wave sent into the cable has travelled to the receiving end before sending another； a series of waves may be sent after each other in rapid succession。 These waves； encroaching upon each other； will coalesce at their bases； but if the crests remain separate； the delicate decipherer at the other end will take cognisance of them and make them known to the eye as the distinct signals of the message。
The mirror galvanometer is at once beautifully simple and exquisitely scientific。 It consists of a very long fine coil of silk…covered copper wire； and in the heart of the coil； within a little air…chamber； a small round mirror； having four tiny magnets cemented to its back； is hung； by a single fibre of floss silk no thicker than a spider's line。 The mirror is of film glass silvered； the magnets of hair…spring； and both together sometimes weigh only one…tenth of a grain。 A beam of light is thrown from a lamp upon the mirror； and reflected by it upon a white screen or scale a few feet distant； where it forms a bright spot of light。
When there is no current on the instrument； the spot of light remains stationary at the zero position on the screen； but the instant a current traverses the long wire of the coil； the suspended magnets twist themselves horizontally out of their former position； the mirror is of course inclined with them； and the beam of light is deflected along the screen to one side or the other； according to the nature of the current。 If a POSITIVE currentthat is to say； a current from the copper pole of the batterygives a deflection to the RIGHT of zero； a NEGATIVE current； or a current from the zinc pole of the battery； will give a deflection to the left of zero； and VICE VERSA。
The air in the little chamber surrounding the mirror is compressed at will； so as to act like a cushion； and 'deaden' the movements of the mirror。 The needle is thus prevented from idly swinging about at each deflection； and the separate signals are rendered abrupt and 'dead beat；' as it is called。
At a receiving station the current coming in from the cable has simply to be passed through the coil of the 'speaker' before it is sent into the ground； and the wandering light spot on the screen faithfully represents all its variations to the clerk； who； looking on； interprets these； and cries out the message word by word。
The small weight of the mirror and magnets which form the moving part of this instrument； and the range to which the minute motions of the mirror can be magnified on the screen by the reflected beam of light； which acts as a long impalpable hand or pointer； render the mirror galvanometer marvellously sensitive to the current； especially when compared with other forms of receiving instruments。 Messages have been sent from England to America through one Atlantic cable and back again to England through another； and there received on the mirror galvanometer； the electric current used being that from a toy battery made out of a lady's silver thimble； a grain of zinc； and a drop of acidulated water。
The practical advantage of this extreme delicacy is； that the signal waves of the current may follow each other so closely as almost entirely to coalesce； leaving only a very slight rise and fall of their crests； like ripples on the surface of a flowing stream； and yet the light spot will respond to each。 The main flow of the current will of course shift the zero of the spot； but over and above this change of place the spot will follow the momentary fluctuations of the current which form the individual signals of the message。 What with this shifting of the zero and the very slight rise and fall in the current produced by rapid signalling； the ordinary land line instruments are quite unserviceable for work upon long cables。
The mirror instrument has this drawback； however it does not 'record' the message。 There is a great practical advantage in a receiving instrument which records its messages； errors are avoided and time saved。 It was to supply such a desideratum for cable work that Sir William Thomson invented the siphon recorder； his second important contribution to the province of practical telegraphy。 He aimed at giving a GRAPHIC representation of the varying strength of the current； just as the mirror galvanometer gives a visual one。 The difficulty of producing such a recorder was； as he himself says； due to a difficulty in obtaining marks from a very light body in rapid motion； without impeding that motion。 The moving body must be quite free to follow the undulations of the current； and at the same time must record its motions by some indelible mark。 As early as 1859； Sir William sent out to the Red Sea cable a piece of apparatus with this intent。 The marker consisted of a light platinum wire； constantly emitting sparks from a Rhumkorff coil； so as to perforate a line on a strip of moving paper； and it was so connected to the movable needle of a species of galvanometer as to imitate the motions of the needle。 But before it reached the Red Sea the cable had broken dow
