In Alexander Bell's primitive
telephone, the double electromagnet was replaced by a single
permanently magnetized bar
magnet having a small coil or bobbin of fine wire surrounding one
pole, in front of which a thin disc of iron was fixed in a circular
mouthpiece. The disc served as a combined diaphragm and armature. On
speaking into the mouthpiece, the iron diaphragm vibrated with the
voice in the magnetic field of the bar-magnet pole,
and thereby caused undulatory currents in the coil. These currents,
after traveling through the wire to the distant receiver, were received
in an identical apparatus. This design was patented by Bell on January
30, 1877. The sounds were weak and could only be heard when the ear was
close to the earphone/mouthpiece, but they were distinct. WIKIPEDIA |