THE INVENT()K8 (>F THK TELEGKAl'IT AND TELEPIIUNE. G55 



stiii»s loi' the conductoj- iiisiihitcd from an raitli plate l»y iiicaiis ortliiii 

 l»aiat'fiii('<l pajxT. so as to uiNc electrostatic capacity. 



The iii\enlion ol' the telephone constitutes one ol" the greatest a(l- 

 \an(;es that have been uuule in telegTa]thic coiniuuiiication. This is an 

 acoustic telegraph, which has tlie very inii)ortaut merit that the audi 

 ble signals are si)oken words, and hence the instruments can be used 

 1)\ anyone who can hear and speak and who undeistands the language 

 in which the message is transmitted. 



It is well known tliat sound is transmitted through the air from the 

 source to the hearer by waves of condensation and rarefaction, which 

 affect the drum ol'the ear. Wheatstone, as early as 1831, showed that 

 these waves could be transmitted from one i>Iace to another, at a mod 

 crate distance, througli wooden rods and afterward conveyed to the 

 ear by the vibrations given to the air l)y the end of the rod. Similarly, 

 vibrations gi\-en to one diaphiagm can he conveyed to another, at a 

 considerable distance, by connecting the two diai)hragius together by 

 a stretched cord or wire. This appears to have been known for several 

 centuries in the central districts of India, and a similar apparatus was 

 described by Hook in 1(!07. .\ similar apparatus is now used and 

 known as the nu'chani(;al telephone. 



To cause the \ ibrati(nis of one diaphragm to produce corresponding 

 vibrations in another <lia])hragm at a distance, through the agency of 

 an electric current, was the i)roblem of the electric telephone. The first 

 to pro[)ose this seems to haxc been Charles liourseul, who, in 18r)4, sug- 

 gested the use of two plates — one at the transjnitting station, which, 

 by the varying pressuic of the air du<' to the sonnd waves, would oj)en 

 and close an electric circuit; while the other was to be acted on at the 

 receiving station by an electro-maguet, through which the coils of the 

 electric curi'ent ]>assed. The \ar.\ing- strength of the electro imignet, 

 (hie to the rapid succession of currents, was thus to ])e taken advantage 

 of to give tlu^ ]U'oper succession of impulses to the receiving diaphragm, 

 in ISCl Pliilij) ib'is. of l-'riedi-ichsdorf, proposed, in a lectuic deliNcrcd 

 belbre the Physical Society of i'^rankfort. to use an instrument, w hich 

 lu^ called a telephone, for tlu' n'|>roduction at a distance of nuisic and 

 human speech. The a]»paratus consiste<l of a stretched nuMnbrane 

 forming part of one side of a box, into which, by means of a mouth 

 ])iece, the sounds could Itc directed. This membrane w as made to open 

 and close an elect riccircuit at each vibration. At the receiving end 

 an electro-magnet, consisting of a thin rod of iion surrounded by acoil, 

 was placed. The succcssi\-e interruptions and closings of this electric 

 <-urrent was, in accordance with a discoxcry made by i)r. l*age. of Salem, 

 Mass.. in ISMT, to jn-oduce sounds of the same pitch as those ol" the 

 sound directed into the l)o\ of the transmitter. 'I'liis method failed for 

 speech, for the simple reason that sjteech has more cliaracl«'rist ics than 

 pitch; and it was only })artially successful tor musical soumls, froni its 



