TELEPHONE. 



707 



iron. We can now disconnect the battery and 

 join the ends of the wire, as in Fig. 2 ; then, 

 if the piece of iron be palled off and stuck on 

 again, a current of electricity will run through 

 the wire every time it is done. Electricity 

 produced in this way is called magneto-elec- 

 tricity, and the current in the wire is said to 



be an induced electric current. If, now, the 

 wire from bar No. 1 (Fig. 3) be extended to a 

 distance, and coiled around another magnetized 

 bar (No. 2), the currents induced in it, by mak- 

 ing and breaking the contact of the piece of 

 soft iron with the first magnet, will simulta- 

 neously affect the magnetism in the distant 

 magnet also. Though the magnets be a mile 

 or many miles apart, the disturbance in one is 

 immediately and equally manifested in the 

 other. 

 But, what is still more remarkable, these in- 



duced currents may be sent through the wire 

 without the actual contact of the soft iron 

 with the steel magnet. If this piece of iron 

 is brought very near to one magnet without 

 touching it, aud then withdrawn, an electric 

 thrill or wave ia induced in the wire which is 

 felt in the distant magnet, just as if the con- 

 tact had been actually made and broken. And 

 so, if we play the piece of soft iron backward 

 and forward, before the magnet, no matter how 

 rapidly or slightly, each motion is felt as an 

 electric pulse in the magnet at the other end. 



We have here the fundamental principle of 

 the telephone. No galvanic battery is em- 

 ployed to furnish an electrical current, a* in 

 the case of the telegraph ; but the currents in 

 the wires are produced by the motions of the 

 piece of soft iron acting on the magnet. Thus 

 far we have represented these motions in a very 

 rude and coarse way, as if the piece of iron 

 were vibrated backward and forward by the 

 hand; but what we have really to deal with 

 is something infinitely more delicate than this. 

 The piece of soft iron of which wo have been 

 speaking, shown at a, Figs. 2 and 3, represents 

 what is called the diaphragm of the telephone. 

 It is a thin, circular sheet of iron, a couple of 

 inches in diameter, held by its rim, and adjust- 

 ed so that its centre comes very close to the 



Fio. 8. 



end of the magnetized bar. And the motions 

 which now concern us are simply the vibra- 

 tions produced in this iron membrane by the 

 beats against it of agitated air. Everybody 

 knows that sounds are propagated through the 

 aerial medium by waves that travel swiftly 

 from their sources, and that we 

 hear them because the waves 

 strike in rapid succession upon 

 the drum of the ear. It is 

 also well understood that these 

 waves differ greatly in their 

 rates, depending upon the ra- 

 pidity of vibration in the sound- 

 ing body ; and, moreover^ that 

 they are very complex, there 

 being waves within waves of 

 various orders in a single tone. 

 It is the special complexity of 

 these wave-systems, in the dif- 

 ferent cases, that gives those 

 pesuliarities of tone that mark different musi- 

 cal instruments and distinguish the voice in 

 different individuals. These waves, started by 

 a person talking, beat against the diaphragm of 

 the telephone and throw it into vibrations. 

 This iron diaphragm acting inductively on the 



magnet originates magneto-electric currents in 

 the wire helix about it, and these travel to 

 another helix encircling the magnet at the oth- 

 er end, and, acting upon that, exert electro- 

 magnetic effects which increase and decrease 

 the strength of the magnet, thus setting it* dia- 



K... I 



phrngrn into vibration. These vihrntlaMOUlTI 

 spond exactly with those of the first dUibrfrra, 

 and the second diaphrajmi is thus roado to re- 

 store to the air in one place what the firrt <NM 

 received from the air in another place. I 

 air-waves, falling on the tympanum of the tat- 



