TELEPHONE 



5738 



TELEPHONE 



as that of the steel magnet. When the current 

 flows in the opposite direction its own mag- 

 netic force is opposed to that of the steel mag- 

 net and, therefore, it weakens the magnetic 

 field (see ELECTROMAGNET). 



Since the current which flows over the tele- 

 phone line wire from the transmitter is alter- 

 nating, the magnetic field is alternately 

 strengthened and weakened. The repeated 

 changing of the magnetic force sets the disk 

 vibrating. When the magnetic pull is stronger, 

 the disk is pulled inward; when the pull is 

 weaker, the disk is released and springs out- 

 ward. The disk in the receiver is thus made 

 to vibrate the same number of times per sec- 

 ond as the disk in the transmitter; for every 

 vibration of the transmitter diaphragm there 

 is produced a similar vibration of the receiver 

 diaphragm. The receiver diaphragm sets the 

 air vibrating and produces sound waves of the 

 same kind as those that are acting upon the 

 transmitter diaphragm. 



The Microphone. A simple instrument which 

 illustrates the working of the telephone trans- 

 mitter can be made of three electric light car- 

 bons. Two of the carbons are laid on a board 

 a few inches apart, and the third carbon is laid 

 across the first two. The first two carbons are 

 then connected to a battery and telephone re- 

 ceiver. If a watch is laid on the board the 

 ticking of the watch can be heard in the re- 

 ceiver. The ticking jars the board and each 

 jar moves the upper carbon just a little and 

 loosens the contact between the carbons. Be- 

 tween the ticks of the watch the upper carbon 

 drops down and presses more firmly on the 

 lower carbons. When the contact is loosened 

 the battery current is made weaker, while a 

 firmer contact makes the battery current 

 stronger; thus as the watch ticks the battery 

 current becomes alternately stronger and 

 weaker. Each time the current becomes 

 stronger the electromagnet in the receiver jerks 

 the metal disk, and then the magnet lets the 

 disk spring back as the current becomes weaker. 

 Thus the disk is jerked back and forth as the 

 watch ticks, and this motion of the disk pro- 

 duces sound in the air, the same kind of sound 

 as the ticking of the watch. 



The Transmitter. The telephone transmitter 

 is a specially designed microphone. Two pol- 

 ished plates of carbon form the front and back 

 of a circular box which is filled with carbon 

 granules. The sides of the box are of insulat- 

 ing material, such as hard rubber. The carbon 

 plate which forms the front of the box is At- 



tached firmly to the center of a metal dia- 

 phragm, which is just back of the mouthpiece 

 (see page 5739). When one speaks near the 

 mouthpiece the sound waves in the air set the 

 disk in vibration; the carbon plate attached to 

 the disk moves in and out a little as the disk 

 vibrates. As the carbon plate moves in and 

 out it repeatedly changes the pressure on the 

 carbon granules, crowding them more tightly 



RECEIVER 



Simple construction, showing parts, such as a 

 boy can master. The parts are (a) wooden pill 

 box; (&) cap; (c) thin, flat spool, wound with 

 copper wire; (d) thin disk of metal; (t) tube; 

 (w) wires; (m) magnet. 



together and releasing them many times a 

 second. The carbon granules form a micro- 

 phone, like the carbon rods, but with many 

 contact points instead of two. A battery cur- 

 rent flows through the carbon granules, and as 

 the pressure on them changes the strength of 

 the battery current changes; for when the 

 granules are more tightly crowded together 

 more current can flow, and when the granules 

 are loosened less current can flow. Thus when 

 the diaphragm vibrates the battery current 

 continually varies in strength. 



The battery current r flows through the pri- 

 mary winding of a small induction coil, which 

 is usually placed in the box back of the mouth- 

 piece. As the battery current in the primary 

 winding varies in strength, an alternating cur- 

 rent is induced in the secondary winding of the 

 induction coil. It is this alternating current 

 which flows along the line wire to the receiver 

 at the other end of the line. This alternating 

 current varies as the sound of the voice varies, 

 alternating more rapidly for sounds of higher 

 pitch than for sounds of lower pitch, changing 

 in strength as the sound changes in intensity, 

 so that the pitch, the intensity and the quality 

 of the sound produce corresponding frequency, 

 intensity and form of electrical vibration. This 

 in turn reproduces sound of the same quality 

 and pitch as the original sound, but of less in- 

 tensity because of loss of power in the line. 



The Switchboard. The switchboard in a tele- 

 phone exchange is for the purpose of making 



