146 Hature'0 /IMracles. 



The construction of the simplest form of 

 telephone is as follows: Take a piece of iron 

 rod one-half or three-quarters of an inch long 

 and one-quarter inch thick, and after putting 

 a spool-head on each end to hold the wire in 

 place wind it full of fine insulated copper 

 wire; fasten the end of this spool to the end 

 of a straight-bar permanent magnet. Then 

 put the whole into a suitable frame, and 

 mount a thin circular diaphragm (membrane 

 or plate) of iron or steel, held by its edges, so 

 that the free end of the spool will come near 

 to but not touch the center of the diaphragm. 

 This diaphragm must be held rigidly at the 

 edges. 



Now if the two ends of the insulated copper 

 wires are brought out to suitable binding- 

 screws the instrument is done. 



The permanent steel magnet serves a double 

 purpose. When the telephone was first used 

 commercially, the instrument now used as a 

 receiver was also used as a transmitter. As a 

 transmitter it is a dynamo-electric machine. 

 Every time the iron diaphragm is moved in 

 the magnetic field of the pole of the perma- 

 nent magnet, which in this case is the free end 

 of the spool (the iron of the spool being mag- 

 netic by contact with the permanent magnet), 

 there is a current set up in the wire wound on 

 the spool; a short impulse, lasting only as 

 long as the movement lasts. The intensity of 



