heroes of the telegraph-第19章

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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needle。　　But　before　it　reached　the　Red　Sea　the　cable　had　broken　down；　and　the　instrument　was　returned　dismantled；　to　be　superseded　at　length　by　the　siphon　recorder；　in　which　the　marking　point　is　a　fine　glass　siphon　emitting　ink；　and　the　moving　body　a　light　coil　of　wire　hung　between　the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　of　the　globe；　from　Northern　Europe　to　Southern　Brazil；　from　China　to　New　England。　　Many　and　complete　are　the　adjustments　for　rendering　it　serviceable　under　a　wide　range　of　electrical　conditions　and　climatic　changes。　　The　siphon　is；　of　course；　in　a　mechanical　sense；　the　most　delicate　part；　but；　in　an　electrical　sense；　the　mouse…mill　proves　the　most　susceptible。　　It　is　essential　for　the　fine　marking　of　the　siphon　that　the　ink　should　neither　be　too　strongly　nor　too　feebly　electrified。　　When　the　atmosphere　is　moderately　humid；　a　proper　supply　of　electricity　is　generated　by　the　mouse…mill；　the　paper　is　sufficiently　moist；　and　the　ink　flows　freely。　But　an　excess　of　moisture　in　the　air　diminishes　the　available　supply　of　EXALTED　electricity。　　In　fact；　the　damp　depositing　on　the　parts　leads　the　electricity　away；　and　the　ink　tends　to　clog　in　the　siphon。　　On　the　other　hand；　drought　not　only　supercharges　the　ink；　but　dries　the　paper　so　much　that　it　INSULATES　the　siphon　point　from　the　metal　tablet　and　the　earth。　　There　is　then　an　insufficient　escape　for　the　electricity　of　the　ink　to　earth；　the　ink　ceases　to　flow　down　the　siphon；　the　siphon　itself　becomes　highly　electrified　and　agitated　with　vibrations　of　its　own；　the　line　becomes　spluttered　and　uncertain。

Various　devices　are　employed　at　different　stations　to　cure　these　local　complaints。　　The　electrician　soon　learns　to　diagnose　and　prescribe　for　this；　his　most　valuable　charge。　　At　Aden；　where　they　suffer　much　from　humidity；　the　mouse…mill　is　or　has　been　surrounded　with　burning　carbon。　At　Malta　a　gas　flame　was　used　for　the　same　purpose。　　At　Suez；　where　they　suffer　from　drought；　a　cloud　of　steam　was　kept　rising　round　the　instrument；　saturating　the　air　and　paper。　　At　more　temperate　places　the　ordinary　means　of　drying　the　air　by　taking　advantage　of　the　absorbing　power　of　sulphuric　acid　for　moisture　prevailed。　　At　Marseilles　the　recorder　acted　in　some　respects　like　a　barometer。　　Marseilles　is　subject　to　sudden　incursions　of　dry　northerly　winds；　termed　the　MISTRAL。　　The　recorder