heroes of the telegraph-第15部分

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f　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　down；　and　the　instrument　was　returned　dismantled；　to　be　superseded　at　length　by　the　siphon　recorder；　in　which　the　marking　point　is　a　fine　glass　siphon　emitting　ink；　and　the　moving　body　a　light　coil　of　wire　hung　between　the　poles　of　a　magnet。

The　principle　of　the　siphon　recorder　is　exactly　the　inverse　of　the　mirror　galvanometer。　　In　the　latter　we　have　a　small　magnet　suspended　in　the　centre　of　a　large　coil　of　wirethe　wire　enclosing　the　magnet；　which　is　free　to　rotate　round　its　own　axis。　　In　the　former　we　have　a　small　coil　suspended　between　the　poles　of　a　large　magnetthe　magnet　enclosing　the　coil；　which　is　also　free　to　rotate　round　its　own　axis。　　When　a　current　passes　through　this　coil；　so　suspended　in　the　highly　magnetic　space　between　the　poles　of　the　magnet；　the　coil　itself　experiences　a　mechanical　force；　causing　it　to　take　up　a　particular　position；　which　varies　with　the　nature　of　the　current；　and　the　siphon　which　is　attached　to　it　faithfully　figures　its　motion　on　the　running　paper。

The　point　of　the　siphon　does　not　touch　the　paper；　although　it　is　very　close。　　It　would　impede　the　motion　of　the　coil　if　it　did。　　But　the　'capillary　attraction'　of　so　fine　a　tube　will　not　permit　the　ink　to　flow　freely　of　itself；　so　the　inventor；　true　to　his　instincts；　again　called　in　the　aid　of　electricity；　and　electrified　the　ink。　　The　siphon　and　reservoir　are　together　supported　by　an　EBONITE　bracket；　separate　from　the　rest　of　the　instrument；　and　INSULATED　from　it；　that　is　to　say；　electricity　cannot　escape　from　them　to　the　instrument。　　The　ink　may；　therefore；　be　electrified　to　an　exalted　state；　or　high　POTENTIAL　as　it　is　called；　while　the　body　of　the　instrument；　including　the　paper　and　metal　writing…tablet；　are　in　connection　with　the　earth；　and　at　low　potential；　or　none　at　all；　for　the　potential　of　the　earth　is　in　general　taken　as　zero。

The　ink；　for　example；　is　like　a　highly…charged　thunder…cloud　supported　over　the　earth's　surface。　　Now　the　tendency　of　a　charged　body　is　to　move　from　a　place　of　higher　to　a　place　of　lower　potential；　and　consequently　the　ink　tends　to　flow　downwards　to　the　writing…tablet。　　The　only　avenue　of　escape　for　it　is　by　the　fine　glass　siphon；　and　through　this　it　rushes　accordingly　and　discharges　itself　in　a　rain　upon　the　paper。　　The　natural　repulsion　between　its　like　electrified　　particles　causes　the　shower　to　issue　in　spray。　　As　the　paper　moves　over　the　pulleys　a　delicate　hair　line　is　marked；　straight　when　the　siphon　is　stationary；　but　curved　when　the　siphon　is　pulled　from　side　to　side　by　the　oscillations　of　the　signal　coil。

It　is　to　the　mouse…mill　that　me　must　look　both　for　the　electricity　which　is　used　to　electrify　the　ink　and　for　the　motive　power　which　drives　the　paper。　　This　unique　and　interesting　little　motor　owes　its　somewhat　epigrammatic　title　to　the　resemblance　of　the　drum　to　one　of　those　sparred　wheels　turned　by　white　mice；　and　to　the　amusing　fact　of　its　capacity　for　performing　work　having　been　originally　computed　in　terms　of　a　'mouse…power。'　The　mill　is　turned　by　a　stream　of　electricity　flowing　from　the　battery　above　described；　and　is；　in　fact；　an　electro…magnetic　engine　worked　by　the　current。

The　alphabet　of　signals　employed　is　the　'Morse　code；'　so　generally　in　vogue　throughout　the　world。　　In　the　Morse　code　the　letters　of　the　alphabet　are　　represented　by　combinations　of　two　distinct　elementary　signals；　technically　called　'dots'　and　'dashes；'　from　the　fact　that　the　Morse　recorder　actually　marks　the　message　in　long　and　short　lines；　or　dots　and　dashes。　　In　the　siphon　recorder　script　dots　and　dashes　are　represented　by　curves　of　opposite　flexure。　　The　　condensers　are　merely　used　to　sharpen　the　action　of　the　current；　and　render　the　signals　more　concise　and　　distinct　on　long　cables。　　On　short　cables；　say　under　three　hundred　miles　long；　they　are　rarely；　if　ever；　used。

The　speed　of　signalling　by　the　siphon　recorder　is　of　course　regulated　by　the　length　of　cable　through　which　it　is　worked。　　The　instrument　itself　is　capable　of　a　wide　range　of　speed。　　The　best　operators　cannot　send　over　thirty…five　words　per　minute　by　hand；　but　a　hundred　and　twenty　words　or　more　per　minute　can　be　transmitted　by　an　automatic　sender；　and　the　recorder　has　been　found　on　land　lines　and　short　cables　to　write　off　the　message　at　this　incredible　speed。　　When　we　consider　that　every　word　is；　on　the　average；　composed　of　fifteen　separate　waves；　we　may　better　appreciate　the　rapidity　with　which　the　siphon　can　move。　　On　an　ordinary　cable　of　about　a　thousand　miles　long；　the　working　speed　is　about　twenty　words　per　minute。　　On　the　French　Atlantic　it　is　usually　about　thirteen；　although　as　many　as　seventeen　have　sometimes　been　sent。

The　'duplex'　system；　or　method　of　telegraphing　in　opposite　directions　at　once　through　the　same　wire；　has　of　late　years　been　applied；　in　connection　with　the　recorder；　to　all　the　long　cables　of　that　most　enterprising　of　telegraph　companiesthe　Easternso　that　both　stations　may　'speak'　to　each　other　simultaneously。　　Thus　the　carrying　capacity　of　the　wire　is　in　practice　nearly　doubled；　and　recorders　are　busy　writing　at　both　ends　of　the　cable　at　once；　as　if　the　messages　came　up　out　of　the　sea　itself。

We　have　thus　far　followed　out　the　recorder　in　its　practical　application　to　submarine　telegraphy。　　Let　us　now　regard　it　for　a　moment　in　its　more　philosophic　aspect。　　We　are　at　once　struck　with　its　self…dependence　　as　a　machine；　and　even　its　resemblance　in　some　respects　to　a　living　creature。　　All　its　activity　　depends　on　the　galvanic　current。　　From　three　separate　sources　invisible　currents　are　led　to　its　principal　parts；　and　are　at　once　physically　changed。　　That　entering　the　mouse…mill　becomes　transmuted　in　part　into　the　mechanical　motion　of　the　revolving　drum；　and　part　into　electricity　of　a　more　intense　natureinto　mimic　lightning；　in　fact；　with　its　accompaniments　of　heat　and　sound。　　That　entering　the　signal　magnet　expends　part　of　its　force　in　the　magnetism　of　the　core。　　That　entering　the　signal　coil；　which　may　be　taken　as　the　brain　of　the　instrument；　appears　to　us　as　INTELLIGENCE。

The　recorder　is　now　in　use　in　all　four　quarters