heroes of the telegraph-第16部分

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his　admirable　inventions；　the　mirror　instrument　of　1857　and　the　siphon　recorder　of　1869；　that　messages　through　long　cables　are　so　cheap　and　fast；　and；　as　a　consequence；　that　ocean　telegraphy　is　now　so　common。　　Hence　some　account　of　these　two　instruments　will　not　be　out　of　place。

Sir　William　Thomson's　siphon　recorder；　in　all　its　present　completeness；　must　take　rank　as　a　masterpiece　of　invention。　　As　used　in　the　recording　or　writing　in　permanent　characters　of　the　messages　sent　through　long　submarine　cables；　it　is　the　acknowledged　chief　of　'receiving　instruments；'　as　those　apparatus　are　called　which　interpret　the　electrical　condition　of　the　telegraph　wire　into　intelligible　signals。　Like　other　mechanical　creations；　no　doubt　its　growth　in　idea　and　translation　into　material　fact　was　a　step…by…step　process　of　evolution；　culminating　at　last　in　its　great　fitness　and　beauty。

The　marvellous　development　of　telegraphy　within　the　last　generation　has　called　into　existence　a　great　variety　of　receiving　instruments；　each　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