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POPULAR SCIENCE MONTHLY 



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Fig. 18. 



believed combines the advantages of the (thirteen) different systems." 

 A diagrammatic representation of the wiring of a single circuit in this 

 board is shown in Fig. 19. There is also shown a flexible cord attached 

 to a plug and a wedge of hard wood having a metal plate fastened to 

 one side. The instructions sent with the board read : 



The local size gravity battery is used — one cell for each bell and for 

 each mile of wire is sufficient. A circuit one mile long having ten bells re- 

 quires about fourteen cells of battery. Two circuits may be operated by one 

 . battery if they are about equal length and 



I have the same number of bells on each. . . . 



K~w3 When any subscriber on this circuit wishes 



to call the central office he presses his knob 

 twice, which rings the bell ; the operator 

 then inserts the wedge between the spring 

 and the plate, with the metal side against 

 the spring, and the plug into a brass strip 

 which is connected through a set of tele- 

 phones to the ground. This, it will be seen, 

 takes off the battery and connects the tele- 

 phones so that the operator can talk with 

 the subscriber and ascertain his wants. If 

 the subscriber wants to talk with a person 

 on another circuit, the central office calls 

 that person and on receiving his answer, 

 the two circuits are connected together by 

 inserting a wedge under each spring and 

 putting each plug into one of a pair of 

 brass strips which are connected together 

 through a hand telephone by means of which 

 the central office operator can ascertain 

 when the two persons have finished using 

 the circuits. Then he removes the wedges 

 and plugs and the circuits are ready for another call. 



The instructions for the subscribers equipment read : 



The circuits are run from the central office and grounded at the last 



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