THE TELEPHONE 



275 



As current flows into the electromagnets, they at- 

 tract the thin diaphragm, bowing it toward them. 

 When the current is lessened the attraction of the 

 electromagnets is lessened and the diaphragm springs 

 back. Vibrations of this diaphragm set the air par- 

 ticles near it in vibration, causing sound waves to be 

 sent out. 



When the varying electrical currents come over the 

 line from the transmitter, a varying attraction is set 

 up in the electromagnets of the receiver. The iron 

 diaphragm is made to vibrate with the variations of 

 the electric currents in the wire. Sound waves are 

 thus sent out from the receiver which are similar to 

 those that were spoken into the transmitter and that 

 caused the varying electric currents or waves to flow 

 over the wire. 



Courtesy Bell Telephone Laboratories 



FIG. 449. SIMPLE TELEPHONE CIRCUIT 



Exercise. Refer to Figure 449 and explain in detail 

 the function of each part shown and how it works to 

 make possible sending the voice by wire. 



What happens at the telephone central? The re- 



Courtesy Bell Telephone Laboratories 



FIG. 450. MODERN TELEPHONE SWITCHBOARD 



markable development of the telephone system would, 

 have been impossible without the invention and de- 

 velopment of the switchboard. This device makes it 

 possible for one to raise the receiver from its hook 

 and be connected with anyone in the same city or a 

 distant city who has a telephone. 



There are several types of telephone switching 

 equipment in use : 



The magneto or local battery system. This type of 

 switching equipment is used largely in rural com- 

 munities because it has been found to be dependable 

 and serviceable. Batteries which furnish current for 

 talking are contained within each telephone. Current 

 for signaling the central operator is generated by a 

 hand generator also contained within the telephone. 

 A turn of a small crank generates sufficient current to 

 signal the operator. 



The central battery system. In this type of switching,, 

 current both for ringing the bells and for talking is 

 furnished from batteries and generators located in the 

 central station. The lifting of the receiver causes a 

 small electric lamp to light on the switchboard in 

 front of the central operator. 



Automatic switching devices. In many large cities 

 automatic devices are being used to displace the cen- 

 tral operator. In this system connection between par- 

 ties is made by several complicated mechanical de- 

 vices. In some places a combination of the operator 

 and automatic systems is in use. 



Figure 450 shows a switchboard in a modern central 

 station. 



Every large city is divided into districts for tele- 

 phone purposes. Each of these districts has a name 

 like "Main," "Central," or "Republic," and each has 

 a central station. Each central station is connected 

 with every other one in the city by several telephone 

 lines called "trunks." 



If you wish to talk with someone located in your 

 telephone district you take the receiver from the hook. 

 This lights a little lamp on the switchboard in front of 

 an operator at the central station. Before her are sev- 

 eral short lengths of flexible telephone cord with plug- 

 like devices called "jacks" on the ends. She inserts a 

 jack in an opening beside the lamp signal and answers 

 you. When you give the number desired she plugs the 

 other end of the flexible cord into the little opening 

 which leads out to the telephone with which you wish 

 to speak. After the jack has been placed in the proper 

 opening, she sends a ring signal over the wire to call 

 your party to the telephone. 



If you wish to talk into another telephone district 

 or to a distant city, the operator connects you over a 

 trunk line with a central office in the district or city 

 to which you wish to speak. An operator in this dis- 

 trict or city then rings the bell of the telephone with 

 which you are to be connected. 



