TELEPHONE 



5740 



TELEPHONE 



Automatic Telephone. This is a system of 

 telephony which requires no operator at the 

 switchboard. A system of electromagnets at 

 the central station makes the connections. The 

 devices for making connections, called selectors 

 and connectors, not only select the number 

 called for but make a busy test, ring the signal 

 at the called station and complete the circuit 

 of a talking current. The talking and ringing 

 circuits are essentially the same as in the 

 switchboard telephone. 



The number is called by means of a dial. 

 The caller's finger is inserted in the hole over 

 the first figure, and the dial is rotated until 

 the finger comes to the finger stop. Then the 

 dial is released and springs back. Then the 



Receiver^ L ; ne ^ ^ Q ^ 9 ^Rece.ver 



Bell 



Bell 



^Batteries 



Batteries 



ARRANGEMENT OF TELEPHONE LINES 

 Two boys can construct a simple line with two 

 telephones, making connections as shown above. 



second figure is taken in the same way, and so 

 on until all the figures of the required number 

 are taken. The dial is connected to a wheel with 

 ten teeth, to correspond to the ten figures on 

 the dial. When the dial is turned to the tenth 

 figure, the wheel by means of its teeth makes 

 and breaks the circuit ten times. This electro- 

 magnet is in the selector which takes the place 

 of a switchboard. Two electromagnets in the 

 selector, both operated by the turning of the 

 dial, make the connections. There are in the 

 selector one hundred contacts, ten rows of ten 

 contacts each, arranged in a semicircular bank. 

 Each contact consists of a free tongue or strip 

 projecting inward toward the center of the 

 curve. A vertical rod at the center carries two 

 springs which can sweep over and under the 

 contact strips, the two springs clasping a strip 

 and making a firm contact. These two springs 

 are called a "wiper." By raising the rod and 

 turning it, the wiper can be made to connect 

 with any one of the 100 contact strips. The 

 use of the electromagnets is to raise and turn 

 the rod to the position required to connect with 

 the number desired. On the rod is a row of 

 ratchet teeth, and the armature of the electro- 

 magnet operates a pawl which turns the ratchet 



wheel. Each time the circuit is closed the 

 electromagnet pulls up its armature and lifts 

 the rod one notch. When the circuit is broken 

 the armature drops, and the pawl catches the 

 next notch, the rod being prevented from slip- 

 ping back by a "dog." 



The first figure of the number called deter- 

 mines the number of notches the rod is raised. 

 For example, if 5 is the first figure, the dial is 

 first turned from 5 to the finger stop. This 

 gives five impulses to the electromagnet arma- 

 ture, raising the rod five notches, and raising 

 the wiper to the fifth row of contacts. Another 

 electromagnet acting horizontally turns the rod 

 on its axis by means of a pawl and a set of 

 teeth arranged in a circle around the rod. For 

 example, if 6 is the second figure, six impulses 

 are given to the horizontal magnet, turning 

 the rod six notches and connecting the wiper 

 with the sixth contact. Connection is then 

 made with the number 56. For an exchange of 

 not more than a hundred subscribers one se- 

 lector for each subscriber makes all the con- 

 nections. If there are more than 100 and not 

 more than 10,000 subscribers, additional sets of 

 selectors must be used. The first- selector acts 

 as a connector, and connects the subscriber to 

 a certain selector in another group of 100 se- 

 lectors. This selector then completes the con- 

 nection as desired. In this case there are four 

 figures in the number called. 



Some Special Applications of the Telephone. 

 The electrophone is a telephone specially ar- 

 ranged to enable subscribers to listen to per- 

 formances in theaters, concert halls, etc. A 

 special transmitter more sensitive than the or- 

 dinary is used. A number of subscribers may 

 be connected to the same transmitter. 



The telephone fire alarm has the advantage 

 over other fire alarm systems in that it not 

 only rings in an alarm signal, but enables the 

 person sending in the alarm to give particulars 

 to the officials. 



In war the telephone is used to keep the 

 commander-in-chief in touch with every por- 

 tion of the army. The line wires are usually of 

 thin, bare, copper wire wound on a reel, one 

 man letting out the wire as he rides along, 

 another following and by means of hooked 

 rods lifting the wire to branches of trees for 

 insulation. The earth is used for the return 

 circuit. A metal rod at each end of the line is 

 driven into the damp ground or into living 

 trees where the sap makes the connection. A 

 buzzer is used for signaling, and the Morse 

 code is commonly used, as such signals are 



