ON SOME MARVELS IN TELEGRAPHY. 2*9 



to traverse the coils whenever it acts, or rather whenever its 

 action changes. If an armature were placed across its ends 

 or poles, at the moment when it drew that armature to the 

 poles by virtue of its magnetic power, a current would 

 traverse the coils ; but afterwards, so long as the armature 

 remained there, there would be no current If an armature 

 placed near the poles were shifted rapidly in front of the 

 poles, currents would traverse the coils, or be induced, their 

 intensity depending on the strength of the magnet, the length 

 of the coil, and the rapidity and range of the motions. In 

 front of the poles of the magnet is a diaphragm of very 

 flexible iron (or else some other flexible material bearing a 

 small piece of iron on the surface nearest the poles). A 

 mouthpiece to converge the sound upon this diaphragm 

 substantially completes the apparatus at each station. Pro- 

 fessor Bell thus describes the operation of the instrument : 

 "The motion of steel or iron in front of the poles of a 

 magnet creates a current of electricity in coils surrounding 

 the poles of the magnet, and the duration of this current of 

 electricity coincides with the duration of the motion of the 

 steel or iron moved or vibrated in the proximity of the 

 magnet When the human voice causes the diaphragm to 

 vibrate, electrical undulations are induced in the coils around 

 the magnets precisely similar to the undulations of the air 

 produced by the voice. The coils are connected with the 

 line wire, and the undulations induced in them travel through 

 the wire, and, passing through the coils of another instru- 

 ment of similar construction at the other end of the line, are 

 again resolved into air undulations by the diaphragm of this 

 (other) instrument" 



So perfectly are the sound undulations repeated though 

 the instrument has not yet assumed its final form that not 

 only has the lightest whisper uttered at one end of a line of 

 140 miles been distinctly heard at the other, but the speaker 

 can be distinguished by his voice when he is known to the 

 listener. So far as can be seen, there is every room to 

 believe that before long Professor Bell's grand invention will 



