heroes of the telegraph-第43章

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l　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　the　other。　　The　instrument　was　said　to　indicate　a　change　of　temperature　equivalent　to　one…millionth　of　a　degree　Fahrenheit。　　It　was　tested　by　Edison　on　the　sun's　corona　during　the　eclipse　observations　of　July　29；　1875；　at　Rawlings；　in　the　territory　of　Wyoming。　　The　trial　was　not　satisfactory；　however；　for　the　apparatus　was　mounted　on　a　hen…house；　which　trembled　to　the　gale；　and　before　he　could　get　it　properly　adjusted　the　eclipse　was　over。

It　is　reported　that　on　another　trial　the　light　from　the　star　Arcturus；　when　focussed　on　the　vulcanite；　was　capable　of　deflecting　the　needle　of　the　galvanometer。　When　gelatine　is　substituted　for　vulcanite；　the　humidity　of　the　atmosphere　can　also　be　measured　in　the　same　way。

Edison's　crowning　discovery　at　Menlo　Park　was　the　celebrated　'phonograph；'　or　talking　machine。　　It　was　first　announced　by　one　of　his　assistants　in　the　pages　of　the　SCIENTIFIC　AMERICAN　for　1878。　　The　startling　news　created　a　general　feeling　of　astonishment；　mingled　with　incredulity　or　faith。　　People　had　indeed　heard　of　the　talking　heads　of　antiquity；　and　seen　the　articulating　machines　of　De　Kempelen　and　Faber；　with　their　artificial　vocal　organs　and　complicated　levers；　manipulated　by　an　operator。　　But　the　phonograph　　was　automatic；　and　returned　the　words　which　had　been　spoken　into　it　by　a　purely　mechanical　mimicry。　　It　captured　and　imprisoned　the　sounds　as　the　photograph　retained　the　images　of　light。　　The　colours　of　Nature　were　lost　in　the　photograph；　but　the　phonograph　was　said　to　preserve　the　qualities　even　of　the　human　voice。　Yet　this　wonderful　appliance　had　neither　tongue　nor　teeth；　larynx　nor　pharynx。　　It　appeared　as　simple　as　a　coffee…mill。　　A　vibrating　diaphragm　to　collect　the　sounds；　and　a　stylus　to　impress　them　on　a　sheet　of　tinfoil；　were　its　essential　parts。　　Looking　on　the　record　of　the　sound；　one　could　see　only　the　scoring　of　the　stylus　on　the　yielding　surface　of　the　metal；　like　the　track　of　an　Alpine　traveller　across　the　virgin　snow。　These　puzzling　scratches　were　the　foot…prints　of　the　voice。