274 



COMMUNICATION 



a second, causes a rapid vibration of the armature and 

 a continuous ringing of the bell. 



Exercise. Devise a method by which the electric bell 

 could be made to give only one tap when the current is 

 turned on, instead of ringing continuously. Suggestion: 

 Experiment with connections at various points on the 

 bell. 



How does the telephone transmitter work? Many 

 people believe that sound is carried over the wire in 

 some mysterious way. Figure 449 shows a simple tele- 

 phone circuit with battery transmitter and receiver. 

 The sound waves, carried by the air, are changed into 

 electric currents by the transmitter. The electric cur- 

 rents, which are streams of negative electrical parti- 

 cles called electrons, are carried through the wire tq 

 the receiver, which changes them back into sound 

 waves like those which entered the transmitter. 



The voice sets up vibrations in the air. As one 

 speaks, the molecules which make up the air are 

 set in vibration by the variations of the voice. If a 

 single tone or a series of tones or words is spoken, 

 there will be a series of places where the air particles 

 or molecules are crowded together, each place being 

 followed by one where the particles are not crowded. 

 Such a series of crowded and uncrowded air molecules 

 makes up a train of sound waves. As sound waves 

 strike the eardrum they set it in vibration. These 

 vibrations are communicated to the brain, and we 

 have the sensation of sound. 



The telephone transmitter is a very sensitive elec- 

 trical ear. Just back of the familiar mouthpiece of the 

 transmitter is a very light and sensitive diaphragm 

 which vibrates much as the human eardrum when 

 sound waves strike it. The first diagram in Figure 

 446 shows how the crowded part of a sound wave 



Courtesy Bell Telephone Laboratories 



FIG. 446. ACTION OF SOUND WAVES ON TRANSMITTER DIAPHRAGM 



pushes the transmitter diaphragm outward ; the other 

 drawing shows the same diaphragm a fraction of a 

 second later when it has sprung back because an un- 

 crowded part of the sound wave cannot push as hard 

 as the crowded part. 



Just behind the sensitive diaphragm is a little box 

 which is partly filled with tiny pieces of roasted car- 

 bon. When the diaphragm is pushed in, these little 

 pieces of carbon are crowded together. When the dia- 

 phragm springs back, they are loosely arranged. A 

 stream of electrons (electric current) from a battery 

 flows continuously through the transmitter and the 



carbon particles. Figure 447a shows an enlargement 

 of a part of the little carbon box, the diaphragm, and 

 the particles. The lines through the particles represent 

 possible pathways for the electrons of the electric 

 current and show that when the particles are loosely 

 packed only a small current can flow through the 

 transmitter. Figure 447b is an enlargement when the 

 diaphragm has been pushed in by the crowded part of 

 a sound wave, thus packing the carbon particles more 

 closely together. This provides many more pathways 

 through the particles for electrons and thus lets more 

 current flow. As the crowded and uncrowded parts of 

 a sound wave move up to the diaphragm, causing it 

 to move in and out, the carbon particles are first 

 close together and then loosely arranged. This allows 

 first a large and then a small number of electrons to 

 get through the carbon grains and sets up an electric 

 current which is varying like the sound wave spoken 

 into the transmitter. 



How does the telephone receiver work? From the 

 experiment at the beginning of this topic you will 

 recall that the receiver has a horseshoe magnet with 



Courtesy Bell Telephone Laboratories 



FIG. 447. ELECTRON PATHS THROUGH LOOSELY AND CLOSELY PACKED 

 CARBON PARTICLES 



an electromagnet wound upon each pole. The horse- 

 shoe magnet holds a thin diaphragm, made of a mag- 

 netic substance, close to the electromagnets (Fig. 

 448). The line wires lead to the electromagnets of the 

 receiver. 



Courtesy Bell Telephone Laboratories 



FIG. 448. CROSS SECTION OF TELEPHONE RECEIVER 



