heroes of the telegraph-第38部分

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m　a　capacious　dam　into　the　pipe。　　Edison　was　acquainted　with　two　experimental　facts　on　which　to　base　the　invention。

In　1873；　or　thereabout；　he　claimed　to　have　observed；　while　constructing　rheostats；　or　electrical　resistances　for　making　an　artificial　telegraph　line；　that　powdered　plumbago　and　carbon　has　the　property　of　varying　in　its　resistance　to　the　passage　of　the　current　when　under　pressure。　　The　variation　seemed　in　a　manner　proportional　to　the　pressure。　　As　a　matter　of　fact；　powdered　carbon　and　plumbago　had　been　used　in　making　small　adjustable　rheostats　by　M。　Clerac；　in　France；　and　probably　also　in　Germany；　as　early　as　1865　or　1866。　Clerac's　device　consisted　of　a　small　wooden　tube　containing　the　material；　and　fitted　with　contacts　for　the　current；　which　appear　to　have　adjusted　the　pressure。　Moreover；　the　Count　Du　Moncel；　as　far　back　as　1856；　had　clearly　discovered　that　when　powdered　carbon　was　subjected　to　pressure；　its　electrical　resistance　altered；　and　had　made　a　number　of　experiments　on　the　phenomenon。　　Edison　may　have　independently　observed　the　fact；　but　it　is　certain　he　was　not　the　first；　and　his　claim　to　priority　has　fallen　to　the　ground。

Still　he　deserves　the　full　credit　of　utilising　it　in　ways　which　were　highly　ingenious　and　bold。　　The　'pressure…relay；'　produced　in　1877；　was　the　first　relay　in　which　the　strength　of　the　local　current　working　the　local　telegraph　instrument　was　caused　to　vary　in　proportion　to　the　variation；　of　the　current　in　the　main　line。　　It　consisted　of　an　electro…　magnet　with　double　poles　and　an　armature　which　pressed　upon　a　disc　or　discs　of　plumbago；　through　which　the　local　current　Passed。　　The　electro…　magnet　was　excited　by　the　main　line　current　and　the　armature　attracted　to　its　poles　at　every　signal；　thus　pressing　on　the　plumbago；　and　by　reducing　its　resistance　varying　the　current　in　the　local　circuit。　According　as　the　main　line　current　was　strong　or　weak；　the　pressure　on　the　plumbago　was　more　or　less；　and　the　current　in　the　local　circuit　strong　or　weak。　　Hence　the　signals　of　the　local　receiver　were　in　accordance　with　the　currents　in　the　main　line。

Edison　found　that　the　same　property　might　be　applied　to　regulate　the　strength　of　a　current　in　conformity　with　the　vibrations　of　the　voice；　and　after　a　great　number　of　experiments　produced　his　'carbon　transmitter。'　Plumbago　in　powder；　in　sticks；　or　rubbed　on　fibres　and　sheets　of　silk；　were　tried　as　the　sensitive　material；　but　finally　abandoned　in　favour　of　a　small　cake　or　wafer　of　compressed　lamp…black；　obtained　from　the　smoke　of　burning　oil；　such　as　　benzolene　or　rigolene。　This　was　the　celebrated　'carbon　button；'　which　on　being　placed　between　two　platinum　discs　by　way　of　contact；　and　traversed　by　the　electric　current；　was　found　to　vary　in　resistance　under　the　pressure　of　the　sound　waves。　　The　voice　was　concentrated　upon　it　by　means　of　a　mouthpiece　and　a　diaphragm。

The　property　on　which　the　receiver　was　based　had　been　observed　and　applied　by　him　some　time　before。　When　a　current　is　passed　from　a　metal　contact　through　certain　chemical　salts；　a　lubricating　effect　was　noticeable。　　Thus　if　a　metal　stylus　were　rubbed　or　drawn　over　a　prepared　surface；　the　point　of　the　stylus　was　found　to　slip　or　'skid'　every　time　a　current　passed　between　them；　as　though　it　had　been　oiled。　　If　your　pen　were　the　stylus；　and　the　paper　on　which　you　write　the　surface；　each　wave　of　electricity　passing　from　the　nib　to　the　paper　would　make　the　pen　start；　and　jerk　your　fingers　with　it。　　He　applied　the　property　to　the　recording　of　telegraph　signals　without　the　help　of　an　electro…magnet；　by　causing　the　currents　to　alter　the　friction　between　the　two　rubbing　surfaces；　and　so　actuate　a　marker；　which　registered　the　message　as　in　the　Morse　system。

This　instrument　was　called　the　'electromotograph；'　and　it　occurred　to　Edison　that　in　a　similar　way　the　undulatory　currents　from　his　carbon　transmitter　might；　by　varying　the　friction　between　a　metal　stylus　and　the　prepared　surface；　put　a　tympanum　in　vibration；　and　reproduce　the　original　sounds。　　Wonderful　as　it　may　appear；　he　succeeded　in　doing　so　by　the　aid　of　a　piece　of　chalk；　a　brass　pin；　and　a　thin　sheet　or　disc　of　mica。　He　attached　the　pin　or　stylus　to　the　centre　of　the　mica；　and　brought　its　point　to　bear　on　a　cylindrical　surface　of　prepared　chalk。　The　undulatory　current　from　the　line　was　passed　through　the　stylus　and　the　chalk；　while　the　latter　was　moved　by　turning　a　handle；　and　at　every　pulse　of　the　electricity　the　friction　between　the　pin　and　chalk　was　diminished；　so　that　the　stylus　slipped　upon　its　surface。　　The　consequence　was　a　vibration　of　the　mica　diaphragm　to　which　the　stylus　was　attached。　　Thus　the　undulatory　current　was　able　to　establish　vibrations　of　the　disc；　which　communicated　themselves　to　the　air　and　reproduced　the　original　sounds。　　The　replica　was　loud　enough　to　be　heard　by　a　large　audience；　and　by　reducing　the　strength　of　the　current　it　could　be　lowered　to　a　feeble　murmur。　　The　combined　transmitter　and　receiver　took　the　form　of　a　small　case　with　a　mouthpiece　to　speak　into；　an　car…piece　on　a　hinged　bracket　for　listening　to　it；　press…keys　for　manipulating　the　call…bell　and　battery；　and　a　small　handle　by　which　to　revolve　the　little　chalk　cylinder。　　This　last　feature　was　a　practical　drawback　to　the　system；　which　was　patented　in　1877。

The　Edison　telephone；　when　at　its　best；　could　transmit　all　kinds　of　noises；　gentle　or　harsh；　it　could　lift　up　its　voice　and　cry　aloud；　or　sink　it　to　a　confidential　whisper。　　There　was　a　slight　Punchinellian　twang　about　its　utterances；　which；　if　it　did　not　altogether　disguise　the　individuality　of　the　distant　speaker；　gave　it　the　comicality　of　a　clever　parody；　and　to　hear　it　singing　a　song；　and　quavering　jauntily　on　the　high　notes；　was　irresistibly　funny。　　Instrumental　notes　were　given　in　all　their　purity；　and；　after　the　phonograph；　　there　was　nothing　more　magical　in　the　whole　range　of　science　than　to　hear　that　fragment　of　common　chalk　distilling　to　the　air　the　liquid　melody　of　sweet　bells　jingling　in　tune。　　It　brought　to　mind　that　wonderful　stone　of　Memnon；　which　responded　to　the　rays　of　sunrise。　　It　seemed　to　the　listener　that　if　the　age　of　miracles　was　past　that　of　marvels　had　arrived；　and　considering　the　simplicity　of　the　materials；　and　the　obscurity　of　its　action；　the　loud…speaking　telephone　was　one　of　the　most　astonishing　of　recent　inventions。

After　Professor　Hughes　had　published　his　discovery　of　the　microphone；　Edison；　recognising；　perhaps；　that　it　and　the　carbon　transmitter　were　based　on　the　same　principle；　and　having　learnt　his　knowledge　of　the　world　in　the　hard　school　of　adversity；　hastily　claimed　the　microphone　as　a　variety　of　his　invention；　but　imprudently　charged　Professor　Hughes　and　his　friend；　Mr。　W。　H。　Preece；　who　had　visited　Edison　at　Menlo　Park；　with　having　'stolen　his　thunder。'　The　imputation　was　indignantly　denied；　and　it　was　obvious　to　all　impartial　electricians　that　Professor　Hughes　had　arrived　at　his　results　by　a　path　quite　independent　of　the　carbon　transmitter；　and　discovered　a　great　deal　more　than　Edison　had　done。　　For　one　thing；　Edison　believed　the　action　of　his　transmitter　as　due　to　a　property　of　certain　poor　or　'semi…conductors；'　whereby　their　electric　resistance　varied　under　pressure。　Hughes　taught　us　to　understand　that　it　was　owing　to　a　property　of　loose　electrical　contact　between　any　two　conductors。

The　soft　and　springy　button　of　lamp…black　became　no　longer　necessary；　since　it　was　not　so　much　the　resistance　of　the　material　which　varied　as　the　resistance　at　the　contacts　of　its　parts　and　the　platinum　electrodes。　　Two　metals；　or　two　pieces　of　hard　carbon；　or　a　piece　of　metal　and　a　piece　of　hard　carbon；　were　found　to　regulate　the　current　in　accordance　with　the　vibrations　of　the　voice。　　Edison　therefore　discarded　the　soft　and　fragile　button；　replacing　it　by　contacts　of　hard　carbon　and　metal；　in　short；　by　a　form　of　microphone。　　The　carbon；　or　microphone　transmitter；　was　found　superior　to　the　magneto…electric　transmitter　of　Bell；　but　the　latter　was　preferable　as　a　receiver　to　the　louder　but　less　convenient　chemical　receiver　of　Edison；　and　the　most　successful　telephonic　system　of　the　day　is　a　combination　of　the　microphone；　or　new　carbon　transmitter；　with　the　Bell　receiver。

The　'micro…tasimeter；'　a　delicate　thermoscope；　was　constructed　in　1878；　and　is　the　outcome　of　Edison's　experiments　with　the　carbon　button。　Knowing　the　latter　to　be　extremely　sensitive　to　minute　changes　of　pressure；　for　example；　those　of　sonorous　vibrations；　he　conceived　the　idea　of　measuring　radiant　heat　by　causing　it　to　elongate　a　thin　bar　or　strip　of　metal　or　vulcanite；　bearing　at　one　end　on　the　button。　　To　indicate　the　effect；　he　included　a　galvanometer　in　the　circuit　of　the　battery　and　the　button。　　The　apparatus　consisted　of　a　telephone　button　placed　between　two　discs　of　platinum　and　connected　in　circuit　with　the　battery　and　a　sensitive　galvanometer。　　The　strip　was　supported　so　that　one　end　bore　upon　the　button　with　a　pressure　which　could　be　regulated　by　an　adjustable　screw　at　the　other。　　The　strip　expanded　or　contracted　when　exposed　to　heat　or　cold；　and　thrust　itself　upon　the　button　more　or　less；　thereby　varying　the　electric　current　and　deflecting　the　needle　of　the　galvanometer　to　one　side　or