28.
Summary.
a. The telephone as invented by Bell in 1875 was crude and limited in efficiency. Extending its area of
usefulness presented many problems. Within only a few years, many of its basic problems were solved.
b. Present-day telephone systems are very different from early ones, but they have the same basic principles of
operation.
c. Sound waves, striking the diaphragm of a carbon transmitter, cause the diaphragm to vibrate with variations
of frequency and amplitude corresponding to those of the waves. This causes a corresponding variation in the resistance
of a chamber of carbon granules, which in turn causes a corresponding variation in the magnitude of a direct current
produced by a battery. This pulsating direct current flows through the primary winding of an induction coil, and induces
an alternating emf in the secondary winding. An alternating current flows through a load connected to the secondary.
d. Modern transmitters are designed, electrically and mechanically, for maximum efficiency and minimum
distortion and interference from external noise.
e. The carbon transmitter is most common of several types of transmitters in current use.
f. The electrical waves produced by the transmitter are sent over the transmission line and reconverted to sound
waves in the receiver.
g. The magnetic-diaphragm receiver contains a permanent magnet which exerts a constant attraction on a
diaphragm of magnetic material placed close to it. Around the magnet is a coil. The intensity of the magnet field of the
magnet in the receiver varies with the alternating current it receives from the transmission line running to the transmitter,
and this causes an alternate increase and decrease in the pull exerted upon the diaphragm. The vibration thus set up
produces sound waves which correspond in frequency and amplitude to both the electrical waves in the line and the
originating sound waves at the transmitter.
h. Modern telephone receivers efficiently reduce interference from surrounding noise.
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