33 2 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



36. The telephone consists of a thin sheet-iron diaphragm D, 

 Fig. 35, which is very near to one end of a steel magnet M with 

 a winding of fine insulated wire C. 



The action of the telephone as a transmitter. When the tele- 

 phone first came into use, the same instrument was used as trans- 



Fig. 35. 



mitter and receiver, being moved alternately from mouth to ear of 

 the speaker. The action of a telephone as a transmitter is as 

 follows : The coil C being near the end of the magnet M, only a 

 portion of the magnetic flux from M passes through the coil. 

 When the diaphragm moves nearer to the end of the magnet, a 

 greater portion of the magnetic flux from the magnet passes through 

 C, and when the diaphragm moves farther away from the magnet, 

 a smaller portion of the magnetic flux from the magnet passes 

 through C. Thus, as the diaphragm D vibrates, the magnetic 

 flux through the coil C increases and decreases. This pulsa- 

 tion of the flux through the coil C induces an electromotive 

 force in the coil, and this induced electromotive force produces a 

 current in the coil and in any circuit to which the coil is con- 

 nected. This induced current flows in one direction while the 

 diaphragm is moving towards the magnet, and in the other direc- 

 tion while the diaphragm is moving away from the magnet. 



The action of a telephone as a receiver. If a current passes 

 through the coil C first in one direction and then in the other 



magnet. But these shocks only sent sparks through the gutta-percha insulating coat- 

 ing and hurried the poor cable to its doom, so that even the three words per minute 

 which would have been the utmost limit of speed possible had this cable been entirely 

 uninjured, were replaced by absolute silence." 



